LOS ANGELES (Reuters) - When millions of Americans turn their faces skyward to witness the nation’s first coast-to-coast total solar eclipse in a century, many will reach for specially designed sunglasses, but experts caution the public to stay clear of unsafe counterfeits.

A solar eclipse is seen from the beach of Ternate island, Indonesia, March 9, 2016. REUTERS/Beawiharta

Even as makers of certified, safety-tested solar eye ware rushed to meet surging demand before the Aug. 21 eclipse, they have joined astronomers and optometrists in warning of defective knockoffs flooding the U.S. market.

“It’s a bunch of unscrupulous people cashing in on the eclipse and putting public safety at risk,” said Richard Fienberg, press officer for the American Astronomical Society (AAS).

Staring at the sun without proper filtration, even when it is partially obscured by the moon during an eclipse, can damage or destroy photo-receptor cells of the eye’s retina, leaving blind spots in a person’s field of vision, experts said.

Special eyeglasses made with proper solar filters allow viewers to safely gaze at the sun any time for unlimited duration, the AAS said.

Although the advent of solar-safe sunglasses dates back more than three decades, they have never been so widely available to the public as for the 2017 event, which Fienberg said may rank as the most watched total solar eclipse in human history.

That is largely because this year’s spectacle will be the first in 99 years to span the entire continental United States - the world’s third most populous nation - across a 70-mile-wide (113-km) path over 14 states, from the Pacific coast of Oregon to the Atlantic shore of South Carolina.

It will also be the first total solar eclipse visible from any of the Lower 48 states since 1979.

SHADY DISTRIBUTORS

As a measure of excitement surrounding the event, a leading supplier of solar lenses, Arizona-based Thousand Oaks Optical, has sold enough of its filters this year alone to produce roughly 100 million pairs of glasses, company owner Pat Steele-Gaishin told Reuters.

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. (here)

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.

The legitimate glasses, which offer no views outside the eclipse, carry their own hazards, however.

“When you put the glasses on, you can look at the sun, but don’t try to walk around,” Chou said.