The nation is preparing for the Aug. 21 "Great American" total solar eclipse, which is the first in 99 years to cross coast-to-coast.
That means buying special eclipse glasses because normal sunglasses – even those with the darkest lenses – aren't enough to protect eyes from damaging rays.
It’s not that the sun is any stronger during an eclipse, but where you would squint, blink and turn away from the full sun on a normal day to protect your eyes, it can be more comfortable to look at the sun as the moon moves over the bright disk.
“A solar eclipse should never be watched the same way we should not stare at the sun,” said Alberto Ortiz, an ophthalmologist with Mittleman Eye in West Palm Beach. “It causes toxicity to the retina and can even cause permanent vision loss.”
Damage to the eye may not be immediately noticeable, but can occur later with blurred vision or complete loss of sight, Ortiz said.
>> Related: Best places to see the 2017 solar eclipse.
Rick Fienberg, the press officer for the American Astronomical Society, said ordinary sunglasses transmit 10 to 20 percent of the light that falls on them.
This makes the landscape on a bright sunny day easier to look at without squinting, and cuts down on glare.
Eclipse glasses allow just 0.0001 percent of the light that falls on them through.
“That’s at least 100,000 times darker than ordinary sunglasses,” Fienberg said. “Nothing can get through such glasses except the sun itself – just enough to be comfortable for viewing.”
The only time it’s safe to look at the eclipse is if you are in the path of totality and the fleeting moments when the sun is completely covered by the moon.
About 12 million people live in the path of totality for the Aug. 21 eclipse. Millions more will travel to get into the path.
“The sun can be viewed safely with the naked eye only during the few brief seconds or minutes of a total solar eclipse,” NASA says on its eclipse website. “Do not attempt to observe the partial or annular phases of any eclipse with the naked eye.”
It is only safe to view a solar eclipse with the naked eye when you are in the path of totality and the moon completely covers the sun.
Proper eclipse glasses are marked with ISO (International Organization for Standardization) and the numbers 12312-2.
Some older solar-viewing glasses may meet previous standards for eye protection, but not the new international standard, Fienberg said.
NASA recommends glasses from the companies Rainbow Symphony, American Paper Optics, Thousand Oaks Optical and TSE 17.
Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate, said it’s a good idea to practice putting the glasses on with children before the eclipse.
“Absolutely make sure the eye protection is clearly on top of the eyes of the child,” Zurbuchen said. “It’s not good to look at the sun when it’s just in the sky and it’s not good when it’s only half or three-quarters covered. The only time it’s OK to look at the sun is when it is entirely covered by the moon.”
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