Recall the goal of this: I want to see how well my eyeglasses, sunglasses, and car windows block ultraviolet (UV) light. See my recent Post #1654 on this topic if you wonder why anyone would care about that.
My test devices arrived from Amazon this afternoon, so I dove right into it. Starting with the simplest test first, using a UV flashlight and a $20 bill. This will only test UV protection at a single frequency — 395 nanometers, the very top-end of UVA range. But that’s enough to get started.
My full set of tools is as follows, detailed in the prior post on this topic:
- A $3 set of cards that discolor in the presence of UV light (Amazon)
- A $6 UV flashlight emitting light at 395 nano meters (Amazon)
- A $40 tanning safety meter measuring combined UVA/UVB (Amazon).
- A new $20 bill to use as the target for the UV flashlight (U.S. Treasury).
This turns out to be far more straightforward than I could have possibly imagined. Using just the cheap flashlight and the $20, and bearing in mind that this only tests protection at the very top-end of the UVA range, I have confirmed that:
- My glasses have almost no UV protection.
- My prescription sunglasses provide good UV protection.
- My wife’s glasses have good UV protection, consistent with a prior test suggesting that they are polycarbonate plastic.
- Toyota front windows block most UV. The rear windows don’t.
- My cheap clip-on polaroids provide excellent UV protection.
Bottom line: A cheap flashlight and a $20 will not give you much in the way of precision. But that’s more than adequate give you a fairly clear yes/some/no indication of the ability of materials to block UVA light.
Details on this simple set of tests, below. Not sure if it’s worth going for more accuracy with the meter.
The setup
I have one UV flashlight (lower left, above), putting out UV radiation at 395 nanometers, just below the top of the UVA band.
Source: UVFAB.com
Ideally, I’d want to test transmission across the full UVA/UVB spectrum. But, in fact, it’s precisely at the top end of the UVA spectrum that most glass and plastics fail the worst. E.g., most window glass does a fine job of stopping UVB, but a poor job of stopping UVA. So a quick test, at this single frequency, isn’t such a bad idea.
I also have a couple of targets that react to UV light: Money, and some UV-sensitive cards. The Amazon references for the flashlight and cards are given in the links above.
And then I have a bunch of eyeglasses and car windows to test.
The main problem is that this is hard to photograph. As you shall see, that turns out to be a big advantage to the UV-sensitive cards, compared to using a $20 bill. Most of what you see below was much clearer by eye, than by camera image.
Test 1: Glasses with no UV protection.
Here are my eyeglasses, first illuminated by the UV flashlight, showing the UV feature on the $20. Note that the UV sensitive strip glows brightly underneath my eyeglass lens. That means my eyeglasses did next-to-nothing to stop the UV radiation.
It’s a lot easier to see this with the UV-sensitive cards.
Throughout, I illuminated the card for 10 seconds, with the UVA flashlight about 1 foot away from the card. Then checked to see how dark the card got. These cards will fade over the course of a couple of minutes, and can be re-used for hundreds of tests.
Consistent with the $20 bill test above, the glasses do nothing to stop UV. The card darkens under the glasses. The only part of the UV-sensitive portion of the card that stayed light was the part under the metal frame.
Test 2: Glasses that actually stop UV.
Same setup, but this time using sunglasses that apparently do stop UV, or at least, the 395 nano meter UV put out by this flashlight. Note that the security feature on the $20 is not glowing under the sunglasses, although in the picture it’s hard to distinguish that, given that the sunglasses are dark. And the UV-sensitive card is not darkened in the area under the sunglasses — much easier to see.
Test 3: Other glasses that stop UV,
including cheap clip-on sunglasses, and my wife’s high-end polycarbonate lens glasses.
Test 4: Toyota 2021 Prius Prime, front windows, rear windows.
All modern windshields stop UV. But only a few cars have side windows that stop UV. In theory, all Toyotas from 2010 and and later have front windows that mostly stop UV. Let me test that with my wife’s 2021 Prius Prime. And yep, the front windows do, in fact, largely stop UV, as the exposed UV-sensitive cards reveal.
Front window:
Rear window.
Conclusion
With this simple and cheap setup, I think I have gotten a pretty good sense of what does and does not stop UV radiation. Using either a $20 bill (hard to photograph) or a UV-sensitive card (easier to photograph), shining a UV flashlight through various media tells a consistent and fairly clear story.
The clear eyeglasses that I feared had no UV protection in fact … had no UV protection. That was evident from the glowing UV security strip on the $20, and the color change on the UV-sensitive card. (Again, when exposed to 395 nanometer UVA light via the cheap UV flashlight.)
On the other hand, the prescription sunglasses, cheap clip-on sunglasses, and my wife’s polycarbonate glasses all appeared to provide good protection from UV. The security strip on the $20 bill didn’t glow (or glow much), and the UV-sensitive card remained light, despite exposure to the UV flashlight.
Finally, the 2021 Toyota Prius Prime appears to provide excellent UV protection for the driver and front-seat passenger. The front windows show vastly reduced UV transmission, compared to the rear (untreated) windows. That’s consistent with the story that Toyota fixed this issue, for the front seat, in 2010.
Arguably the most important conclusion is that you can easily check the UV protection of your own glasses, sunglasses, and car window glass. A cheap UV flashlight, and a set of UV-sensitive cards or a $20 bill, is enough to let you know which substances block most UV, and which let it pass.
Ideally, I’d continue along and do something a little more quantitative. I.e., what fraction of UVA/UVB is being blocked. I may try that on the next sunny day, using the tanning meter I bought. But I already know enough to know that I need to replace my eyeglasses. Which is really where I was headed with this all along.
