While no data exists for how many made-for-eclipse eyeglasses are in circulation overall, shady distributors of purportedly solar-safe shades abound on the Internet, Fienberg said.
The lenses of some obvious fakes allow the penetration of light from such relatively faint sources as fluorescent lamps, while the only thing one should see through authentic solar-safe filters when looking at objects fainter than the sun is pitch blackness.
Other bogus glasses have come stamped with forged logos of reputable manufacturers or with phony safety labels.
Peru State College in Nebraska, which lies in the path of the eclipse, ordered 7,500 pairs for students but discovered the shipment from a Chinese distributor was defective and came with safety certificates for ordinary sunglasses.
Peru marketing director Jason Hogue said the college has since reordered from a legitimate supplier.
The AAS and NASA have posted a list of reputable solar filter brands, retail distributors and online dealers. (https://eclipse.aas.org/resources/solar-filters)
Prices range from as little as 99 cents for a pair of paper-frame glasses to $20 or $30 for a more stylish plastic set.
While a last-minute rush has left many dealers out of stock two weeks before the big day, the good news is that U.S. astronomy buffs have to wait only seven more years for the next total solar eclipse over North America, in April 2024.
Made from an extremely opaque black polymer film containing fine carbon powder, true solar-safe lenses are designed to screen out 250,000 times more visible light than would otherwise reach the naked eye, said B. Ralph Chou, a Canadian optometrist who led development of global standards for solar optics.
Any filter less opaque than that may cause severe eye damage that would not become evident until hours later, Chou warned.