As excitement on social media about the Aug. 21 solar eclipse heats up, experts are urging people to take good care of their eyes when they enjoy the spectacle in Canadian skies.
Ralph Chou, a University of Waterloo optometry professor and president of the Royal Astronomical Society of Canada, says that staring at the sun for more than a few seconds will cause harm.
He says the eye feels no pain so it will be too late to look away from potential retina-burning solar rays before a person notices they've done potentially-permanent damage.
"The problem with the back of the eye is that there's no pain sensors, so all this damage can occur without you even knowing about it until it's far too late ... the next morning they wake up and the photo receptors at the very centre of their vision are damaged and they suddenly realize they can't see their faces in the bathroom mirror," he said.
The person will have to wait an anxious three months to know whether the damage is permanent, Chou said.
Regular sunglasses just won't cut it during the direct viewing for an eclipse because they allow in thousands of times more sunlight than is safe to reach your eyes. NASA says special solar eclipse glasses should be marked with the "ISO 12312-2" international safety standard on the label. Make sure there are no scratches on the lenses.
These glasses cost only a few dollars but are becoming hard to find as we get closer to eclipse date.
This week, Amazon pulled potentially shady glasses from its site and issued refunds to customers who had already purchased them. In an email to buyers, the company said it could not get confirmation from the supplier that the glasses came from a recommended manufacturer.
"We recommend that you DO NOT use this product to view the sun or the eclipse," the email said.
The American Astronomical Society has a list of reputable sellers of solar eclipse glasses on its website. The Royal Astronomical Society of Canada, some universities and science centres will be distributing free eclipse glasses at viewing parties across Canada. Science museums and stores that sell telescopes may also have stock left.
With ads for eclipse glasses for sale on Kijiji and social media it can be hard for armchair astronomers to track down where they were made and whether they meet the standards recommended by NASA.
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Testing eclipse glasses
The Canadian Press asked the University of Toronto to test two pairs of eclipse glasses using an ellipsometer. The machine sends a wave of UV and visible light though the glasses' lens and uses a sensor to measure how much of the light makes it through.
The first pair, bought at one of the stores on the American Astronomical Society list, blocked out more than 99.9 per cent of UV and visible light. The pair was labelled as conforming to the ISO 12312-2 standards for direct observation of the sun.
The second pair, purchased online did not have the ISO mark and did not perform as well. It let through more than 0.1 per cent of visible and UV light.
The ISO standard allows a maximum of 0.0032 per cent of light to be allowed to pass through. The equipment used could not give a sensitive enough reading to verify if the glasses conformed to the standard.
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In both cases, the machine was not able to determine if the glasses met the ISO 12312-2 standard. Both pairs did let through less than one one-thousandth of the light that you'd see with the naked eye.
"Results show us that the two sets of glasses are most likely safe for observing the sun," said Herman Wong, a University of Toronto engineering and photonics PhD student who tested the
glasses. He said the machine found that there was almost zero transmission of light that passed through the lenses, but added that there's really no way to know if they meet NASA recommendations, especially if they are not labelled.
"They are not standardized by any means so use at your own risk," he said.