UV protection from spectacles
Better than you might be led to believe ...
In their blurb, the manufactures of sunglasses like to stress how much they protect your eyes against UV damage. It’s all part of their pitch to sell them at exorbitant prices. But your bog-standard prescriptions glasses may be just as good. Or even the cheapest plastic ones (but I wouldn’t trust those without testing them because some materials do transmit some UVB).
The plot below shows show the UV transmission of several eyeglasses, including prescription eyeglasses and inexpensive plastic eyeglasses.
The main difference between the sunglasses and the others is their reduced transmission in the visible. There’s hardly any difference at UV wavelengths, especially in the most-damaging UVB region where the transmission is essentially zero for all.
Some of those examples are laminated glass. Non-laminated glass typically transmits down to shorter wavelengths. I’m not sure whether or not eyeglasses are normally laminated, but in a sample of about 10 that I tested among my colleagues at work, all blocked at least 98 percent of the sunburning radiation (i.e., they had an equivalent blocking factor of at least SPF50). And that included a pair of cheap reading glasses with plastic lenses. Since at least 5 percent of sunburning radiation is from UVA wavelengths, that transmission of less than 2 percent implies that just about all of the UVA (as well as the UVB) has also been blocked. If they didn’t block any UVA, that protection factor couldn’t exceed SPF20.
Whatever glasses you wear, you do have to be aware of any light that passes around their edges. Wrap-around glasses will be a lot better than those pince-nez style specs.
Richard, according to NIWA the maximum summer value for UVI in New Zealand is generally about 12. Published data from Lauder shows that Ozone atmospheric concentration is lowest in March and the latest measurement for March 2024 is close to the historical average for this time of the year. However, Whangarei recorded only 7.8 maximum today 22nd March and yesterday 21st. Why were these measurements for Whangarei today and yesterday much lower than 12?
This is great info, I was actually just looking into this. I read that there is a difference between prescription glasses using glass or using plastic. Plastic blocking UV rays and glass not really blocking them. I wonder if your test implies that all these glasses were made of some sort of plastic, or if there also we're actual glass lenses among them which only block UV rays due to whatever custom coating they had.
Furthermore I noticed the remark about SPF 50 implying UVA has to get blocked as well.
This is news to me, I understood SPF to only refer to UVB rays. Could you elaborate on the connection here?
Can you make any kind of deductions about the amount of UVA being blocked from the SPF value? Since sunscreen manufacturers don't tell us about the amount of UVA being blocked specifically.