I’d like to know the level of protection from ultraviolet rays that my current eyeglasses and sunglasses provide. In this post, I don’t actually do the test, but I set up all the background information. The test has to wait for materials to arrive from Amazon.
The damage done
If you are reading this, I assume you know why you want to protect your eyes from the ultraviolet (UV) radiation in sunlight. Among other things, exposure to the UV in sunlight is associated with the eventual development of cataracts. Evidence further suggests that it speeds age-related degeneration of your retinas. And so on. (e.g., reference).
Source: UVFAB.com
At the earth’s surface, 95% of the UV in sunlight is “soft” ultraviolet, or UVA. Only 5% is the “harder” UVB. (reference) All available evidence suggests that both ranges of ambient UV light are damaging to the eyes (reference). That said, the more energetic UV-B photons are believed to be much more damaging per unit of light, and the medically-oriented UV Index (UVI) over-weights UVB relative to UVA.
Finally, for all intents and purposes, “hard” ultraviolet (UV-C, the kind used in germicidal lamps) does not penetrate the atmosphere.
Even the visible blue light directly adjacent to the UV spectrum has been implicated in macular degeneration (reference), the most common cause of blindness in the elderly. Basically, everything in the spectrum from blue and above causes some degree of damage to your eyes.
A short road detour.
I’m going to assume that you don’t have glass lenses in your eyeglasses or sunglasses. Where I buy my glasses, glass lenses haven’t been an option for decades. Choices are limited to various types of plastic and coatings.
But eyeglasses aside, the UV transparency of ordinary glass matters greatly for the average American. Common (soda-lime) glass does a reasonable job of stopping shorter-wavelength UV-B radiation. But most glass does only a mediocre job of stopping longer-wavelength UV-A radiation. This is why, for example, fabrics and wood fade if situated near a window. They still get significant UV exposure through the glass.
In the popular press, the UV transmission of ordinary glass is most commonly presented in terms of the dose of UV you get while driving your car.
Here’s a weird fact: “By law, front windshields are treated to filter out most UVA, but side and rear windows generally aren’t.” (Source: skincancer.org).
With that in mind, in a country where the average worker commutes about an hour a day by car, the rest of these facts simply fall into place. “… cataracts tend to occur most often in drivers’ left eyes”. (Source: Motorbiscuit). Or, similarly, “Because drivers in the U.S. have their left side exposed to sunlight, UV rays have been blamed for the increased number of cataracts and skin cancers that occur on the left side.” (Source: Reuters). Or, most grimly, “Studies estimate that 74 percent of melanomas appear on the left side, …” (Source: University of Utah).
Those of you who drive on the wrong side of the road, substitute right for left.
How much UV do car windows stop? Here are couple of averages:
This apparently varies widely by make and model, window thickness and color, and so on. For example, more than a decade ago, Toyota simply decided to fix this problem, so that the front windows of Toyotas now incorporate a film that blocks 99% of incoming UV (source) (source).
Separately, there are many choices of aftermarket window tinting film that claim to block nearly all incoming UV radiation. 3M makes one version for car windows that is almost colorless (reference). So if this is an issue for you, you can fix your car, after-the-fact, so that the windows stop 99+% of UV light. Separately, Home Depot sells clear window film for the home that claims to stop 99% of UV transmission (reference: Home Depot).
Back to the core question: How can I tell if my plastic glasses/sunglasses/window films stop ultraviolet radiation?
I’m assuming you can’t just ask your optician what they made your lenses out of. That’s my particular problem. I have been unable to contact the place that made my glasses, to ask about UV blocking.
Alternatively, I assume you’re interested in testing (e.g.) off-the-shelf sunglasses, or maybe your car windows, and so don’t necessarily believe sellers’ claims about blocking UV. So this isn’t something you could reasonably ask your optician to test for you.
So, do my glass, sunglasses, car windows, and so on block UV or not?
How can you know? How can you test it?
Turns out, this question has been asked before. But as with so many D-I-Y answers you get on the internet, a lot of what you read is less than totally helpful and/or just totally made-up.
D-I-Y measurement of UV transmission
You could always buy a good-quality UV meter. That’s just a light meter that’s sensitive to UV instead of visible light, and may or may not have a way to “tune in” certain bands of UV. Judging from what I see on Amazon, a reliable UV meter is going to run about $250 (e.g., this one). You could spend maybe $75 less and still get what appears to be a good-quality meter, this time from Sper Scientific.
Somewhat more cheaply, you could consider single-purpose meters designed for testing car window tint. There, for maybe $110, you would get a device that could test a window or a lens for transmission at a single UVA light frequency (reference).
That said, arguably the most promising/cheapest sector of the UV-meter market is that oriented toward suntan, and sunburn avoidance. These meters claim to measure the UV Index (a weighted combination of UVA and UVB frequencies). Plausibly, that, plus sunlight, would let you estimate the fraction of UV of all frequencies that is being blocked by your lenses/glass/film. A slight drawback is that these should ideally be used in direct sunlight.
Source: Amazon
In theory, this $40 meter would let you check the ability of lenses, glass, or films to reduce the intensity of UV in sunlight (reference). By measuring the UV Index, it should, in theory, give you a broad-brush measure of the amount of UV energy capable of causing biological damage.
While I would not expect a lot of accuracy from a $40 UV meter, it’s probably good enough to show the reduction that you get when sunlight passes through lenses, glass, or plastic. That is, even if it’s off by 20%, as long as it’s consistently off by 20%, you’ll get an accurate assessment of the reduction in UV from a lens, window, or film.
Indirect measures of UV transmission.
But we can go cheaper still. A commonly-mentioned test is to shine a UV flashlight through your lenses, onto something that glows under UV. Apparently, the “new” $20s have a strip that will glow green when exposed to long-wave (around 375 nanometer wavelength) UV. So you can use a new $20 as your target. Plausibly, if your lenses block UV near that wavelength, the strip will stop glowing as you pass the UV light through your lens.
Source: Amazon
This surely isn’t a quantitative test, but it’s certainly easy enough and cheap enough. Amazon has UV flashlights at the 395 nanometer frequency for around $6, batteries included. That’s at the very upper end of the UV-A band, which ends at a wavelength of 400 nanomaters.
Even cheaper: Use sunlight and a UV-sensitive card. You can find a variety of simple UV-sensitive “test cards” on Amazon. Mainly, these are used for testing lights used for reptile cages, as many reptiles require some UV-B radiation to maintain their health. Here, on Amazon, you can buy a set of cards that is broadly sensitive to UV-A and UV-B, for under $3. In theory, these can give you a semi-quantitative test. The more intense the UV that falls onto the card, the darker it gets, and a calibration scale is shown on the card.
Source: Amazon
For lenses only: Test whether they are polycarbonate.
Part 1: From garden frost protection to polycarbonate lenses.
Last spring, I worked out why mason jars provide excellent frost protection in the garden, but polyethylene sheeting provides none at all. It all has to do with the ability of a material (glass, plastic) to block infrared (IR, long-wave) radiation welling up out of the soil. Common soda-lime glass is almost opaque in those IR frequencies, while polyethylene sheet is perfectly transparent to them. The glass acts as a radiant barrier, the plastic does not. Details are in these posts:
- https://savemaple.org/2021/04/19/g21-014/
- https://savemaple.org/2021/04/19/g21-015/
- https://savemaple.org/2021/04/20/g21-017/
- https://savemaple.org/2021/04/23/g21-018-2/
- https://savemaple.org/2022/03/27/g22-005/
- https://savemaple.org/2022/03/29/g21-006-2/
- https://savemaple.org/2022/03/29/g22-007/
- https://savemaple.org/2022/03/30/g22-008/
That starts out by testing actual, literal radiant barrier — the stuff sold in hardware stores for use in attic spaces. The final shows that plastics vary widely in terms of the frequencies of radiation that they will and will not transmit. While polyethylene provides almost no frost protection, PET/PETE plastic works pretty well. And that’s due entirely to the difference in their abilities to absorb radiation in the long-wave IR portion of the spectrum.
Source: Figure 9, “Identification of black microplastics using long-wavelength infrared hyperspectral imaging with imaging-type two-dimensional Fourier spectroscopy“, Kosuke Nogo, Kou Ikejima, Wei Qi, et al., DOI: 10.1039/D0AY01738H (Paper) Anal. Methods, 2021, 13, 647-659
Which brings me to polycarbonate. I had always assumed that plastic greenhouses used polycarbonate panels because they are tough. But that’s only part of the story. In addition, polycarbonate also serves as a pretty good radiant barrier. It fairly strongly blocks the transmission of long-wave IR. All other things equal, a greenhouse made of polycarbonate panels will stay warmer at night than one covered with untreated polyethylene film.
I had further assumed that some sunglasses and eyeglasses used polycarbonate because it was tough. It makes for a thin but shatter-resistant lens. And, as with greenhouses, that’s true, but there’s more to it. Turns out, polycarbonate is completely opaque to ultraviolet (UV) radiation. If you want eyeglasses or sunglasses that provide complete protection against the UV in sunlight, then polycarbonate lenses will do that. Otherwise, if you want UV protection, and you are using plastic lenses, you need to get an additional UV-opaque coating applied to the lens.
Part 2: View your lenses through of polaroid sunglasses.
As noted above, polycarbonate is opaque to UV. But determining whether or not your lenses are polycarbonate can be a bit of a trick. Of all the (non-destructive) tests I came across, the easiest seemed to be to view them in polarized light or through a polarized lens. Rumor has it that polycarbonate lenses will show “birefringence”, faint rainbow-colored lines that match stress points in the lens. Other common lens plastics will not.
At least, that’s what they say.
This appears to be true. The picture below is daylight passing through two different sets of eyeglasses, viewed through polarized sunglasses. The ones on the left are polycarbonate, the ones on the right are some unknown material.
The picture doesn’t do it justice. In real life, the bifrefringence is clear and pronounced, and is visible from all angles. Here’s the same photo, below, enhanced to emphasize the birefringence pattern.
If you want to see a better picture of birefringence in polycarbonate lenses, try this Reddit thread. Ignore the discussion, as almost all of it appears to be incorrect. The glasses in question are, however, polycarbonate.
In any case, that test matches what I believe I know about the two sets of eyeglasses. On the left are my wife’s glasses — quite expensive, with “thin” lenses, typically made of polycarbonate. On the right are my glasses, almost surely the cheapest pair I could order. I’m pretty sure I didn’t pay extra to have polycarbonate.
I can see an occasional flash of color out of the non-polycarbonate lenses. Plausibly that’s due to the lens coating or to variations in the thickness of the lens. But only the polycarbonate ones have that pronounced, stable birefringence pattern, with lines of color joining points on the edge of the lens.
If your lenses are polycarbonate, you’re done. If they’re not, they might still have a UV-blocking coating applied to them. So you’re not done.
Conclusion
You have plenty of options for testing the UV light transmission of common household objects such as eyeglasses, sunglasses, or windows.
For free, you can examine your eyeglasses or sunglasses to see whether the lenses are polycarbonate. If so, you’re good to go. Those will be opaque to UV.
For $3, you could use UV test cards to determine the intensity of UV in sunlight, versus sunlight that passes through a lens or pane of glass. The depth of color on the card corresponds to the intensity of UV striking it. I would class this as a semi-quantitative method. Using the calibration supplied on the cards, you ought to get a rough estimate of the reduction in UV.
For $6, you could get a yes/no test of unknown quality, for transmission of UV at the 395 nanometer frequency — the top of the UVA band. Shine a UV flashlight through a material and see how brightly it lights up the UV security feature of a new $20 bill.
For around $40, you can get a quantitative test of unknown quality by using a sun-tanning meter. This is a meter that claims to show the UV index, a broad measure of combined UVA/UVB radiation. This would have to be used by shining sunlight directly through the object to be tested.
For $200 and up, you can get a higher-quality UV meter that shows average UV intensity across the UVA and UVB bands. Plausibly these meters are sensitive enough that you could use them with indirect (ambient) sunlight.
When all is said and done, the $40 sun tanning meter seems to be the right level of investment in this, for me. It’s going to cost me a few hundred dollars to replace my eyeglasses and prescription sunglasses if they do not provide adequate UV protection. I’d like to be fairly confident of that before I proceed. I think that an actual meter reading of the UV level — even off a cheap meter — should be enough to let me know whether or not those glasses need replacing.
I shall try a few of these methods and report back next week.
