(CNN) Summer's almost here, so beware the brightest of sunlight! To protect your eyesight, the most important recommendation you need to follow is to wear sunglasses that block ultraviolet radiation whenever you go outside during daylight hours, according to the National Eye Institute. This is true for everyone, no matter what age, year round.
Ultraviolet radiation is the energy radiated by the sun that arrives on Earth in wavelengths too short for us to see. Both UVA (waves that are 320 to 400 nanometers long) and UVB radiation (290 to 320 nanometers long) can be harmful to your eyes. The fix, though, is simple.
"The recommendations are that eyeglasses should block UVA and UVB radiation," said Dr. Andrea Thau, president of the American Optometric Association. When shopping for sunglasses, look for a tag or label that says 100% protection against both UVA and UVB or 100% protection against UV 400.
The UV 400 designation simply means the lenses will block radiation equal to or shorter than 400 nanometers, which covers both UVA and UVB rays, Thau said.
Thau and Dr. Justin Bazan, a doctor of optometry and medical adviser to The Vision Council, a nonprofit trade organization for optical industry manufacturers and suppliers, recommend purchasing sunglasses from a reputable retailer.
These include "eyecare provider offices, or brick-and-mortar and online department stores and sunglass specialty shops -- as they offer sunglasses that meet the necessary standards for proper UV protection," Bazan wrote in an email. He adds that shoppers "should be wary when purchasing sunglasses from online auction sites, street vendors and flea markets, as sunglasses available at these locations may not meet the necessary standards for proper UV protection."
That's all good, but does UV protection wear off over time?
It doesn't, says Dr. Jeff Pettey, assistant professor of ophthalmology at the Moran Center University of Utah and a spokesman for the American Academy of Ophthalmology.
"The UV protection is embedded," Pettey said, explaining both the technology and the process have changed over time, advancing far enough that "routinely, even on the cheapest pair of glasses," the protection is built in for life.
"In the testing we've done, we've never had a pair of sunglasses that didn't meet that UV protection," Pettey said.
He acknowledges that in the early 1990s, tests on children's sunglasses showed that not all lived up to their UV protection claims, but more recently, "we just have not seen that."
Still, he suggests buying from a reputable retailer just to be safe.
"There's no guarantee, because you can't say for certain where your glasses are coming from," Pettey said, adding that there's a test you yourself can perform at any local optical shop that has a UV light meter.
"You can take your glasses in and have them tested," said Pettey. This is a handy test for when you doubt your sunglasses have the UV protection claimed by a retail tag or if they're simply old and you want to make sure.
As far as a "hard requirement," UV protection is it, he said. Tint doesn't matter, polarization doesn't matter, and although bigger is always better, "UV protection is the essential piece."