The modern eclipse glasses had their origins in the 1970s, when amateur astronomers began experimenting with metalized Mylar, a biaxially oriented polyester film developed by DuPont Co. At that time, a big use of the metalized film was to protect seedlings as ground cover in agriculture, Chou recalled. Viewers were preparing for the 1979 U.S. total eclipse.
Chou has seen the evolution of plastic eclipse glasses from these early days. For 30 years he was an astronomy professor at Canada's University of Waterloo until he retired in 2012. Since then he has been a consultant to most major U.S. eclipse glasses producers. He was in a group that devised an international safety standard for eclipse glasses, the ISO-12312-2 standard adopted by the major solar glasses manufacturers. As president of the Toronto branch of the Royal Astronomical Society of Canada, he won't miss the event.
During the 1980s, black-loaded polymer films cropped up as competitors to metalized BOPET film. A disadvantage of the metalized film was it presented solar rays in bluish-white shades. The black films, however, cast the light in orange and yellow shades, which "are a lot more pleasing to the eye," Chou explained.
For years, controversy swirled around the plastic lens choices, with proponents claiming the rival material was subject to pinholes or other defects. Today, the common affordable glasses combine metalization and carbon loading in the polymer film. The composition of the polymer also evolved to overcome manufacturing drawbacks of conventional, thin BOPET film. The early thin films were difficult to mount into the glasses' frames. The search for alternatives faced stringent requirements. The treated polymer film must be dimensionally stable and scratch-resistant. The polymer must disperse the carbon black particles well.
Professional astronomers, not surprisingly, opt for higher tech eclipse viewing options. One standard filter type has a mixed-metal film deposited under vacuum on glass. These filters are far more expensive than the mass-market polymer film types.
Electric arc welding glasses have also been used to view solar eclipses. The glasses can be a thick polycarbonate film with proprietary dyes that block out harmful UV, infrared and visible rays. These filters are expensive and difficult for an amateur to buy. A "shade 14" welding filter is often recommended.
Chou said he's amazed at how important plastics have become in the whole range of optical applications.
"In the past few years, the plastics industry has been very creative in coming up with new materials," Chou said. Plastics are choice materials for prescription lenses, eyeglass frames and protective eyewear.
"There is a huge range of plastics for spectacles now," he said.
On Aug. 21, the eclipse shadow is due to hit landfall at 12:16 p.m. Eastern Time at Depoe Bay, Ore., and exit the country at 2:47 p.m. ET at Awendaw, S.C. Along the total eclipse's 70-mile wide path each viewpoint will experience about 2½ minutes total solar darkness.