Beware sunglasses for little kids!

OK, after the scary title the rest of this post will be boring and technical. Just a little.

Some time ago I remember listening to the radio and hearing about how sunglasses for little kids can be hazardous to their health. The general idea was that the cheap sunglasses that they make for little kids do not block out UV radiation and can end up damaging your kid's eyes. I was slightly skeptical about that, but since I did not own sunglasses intended for little children, nor did I plan on buying a pair any time soon, I did not think much about it.

But for the last few years I have been a TA for the advanced experimental techniques class here at UNC. Specifically I have been teaching labs about optical absorption and Faraday rotation (more on that in another post). As part of my teaching I get to use a spectrophotometer to measure the absorption of a substance or film and from that determine physical properties of the materials (a spectrophotometer is a device that can measure the amount of light absorbed by a material at different wavelengths).

As part of the lab I would have the students compare laser safety goggles to sunglasses in order to show why it is a very bad idea to use sunglasses as laser safety goggles. A few weeks ago my son found a pair of plastic, pink, kid's sunglasses at a playground. They were the kind that kid's might get as a cheap party favor from a birthday party, and as they were already broken and no one would be missing them, I decided to take them to the lab and have them tested to see if cheap sunglasses are as bad as they say they are. I was surprised to find out that it really is possible to damage kids eyes with bad sunglasses.

Below I have a graph of the absorption of the sunglasses at different wavelengths. I also included two pairs of adult sunglasses to give you an idea of what the spectra of "normal" sunglasses look like.

Black--Kid's sunglasses; Blue--Adult Pair #1; Red--Adult Pair #2 (Blue Blockers)

The x-axis is wavelength (in nm, visible is from ~400 - ~700 nm). The y-axis shows absorption coefficient (in a log scale, so A=1 means it only transmits 10% of the light. A=2 lets through 1%, A=3 0.1% etc.). You will notice that the adult sunglasses (the red and the blue lines) not only absorb more light in the visible spectrum but they also take a sharp turn upwards before they get to the UV ( <400 nm). You may also notice that the kid's sunglasses only take a sharp turn upwards (i.e. absorb more light) after 400 nm.

So how is this hazardous to kid's health? Well because they block some of the light (about 70%) if your child were wearing them then their pupils would dilate to compensate. So the radius of the pupil dilates to let in more visible light, but for that small sliver of UVA radiation the glasses do not give any added protection. So the end result is that you get more UV radiation in your kids eyes than they normally would get, because usually you when you have a lot of UV radiation, you also have a lot of visible light, so your pupils contract to let in less light. But by filtering a lot of visible light and adding more protection in the UV you actually end up with more UV radiation in your eye than you would without the sunglasses. In the end, the excess radiation can cause damage and ultimately cause blindness.

While sunglasses are technically regulated so as to avoid this problem for some reason the cheap (and cheapest) glasses that kids get never quite get up to the level that the basic glasses have for adults.