Checking eclipse glasses

The physical inspection

Before you look toward the sun, you must examine the physical frame of your cardboard viewers. Hold the glasses up to a regular overhead light and inspect the black polymer material for any tiny pinholes, fine scratches, or wrinkles. Even a microscopic tear in the thin aluminized layer can let in a dangerous amount of infrared and ultraviolet radiation — both of which can cause permanent blind spots. If you notice any separation between the cardboard frame and the film, or if the film has become crinkled or loose, discard the pair immediately.

The household light test

A certified safe pair of eclipse viewers should block out almost all visible light. To test this, put the glasses on indoors and look around. You should see absolutely nothing. “If you can see ordinary room details, the glasses are unsafe,” says optical safety analyst Sarah Higgins. The only thing that should register through a legitimate ISO-certified filter is a very faint, dim orange or white outline of an extremely bright source, such as the bare filament of a 100-watt incandescent bulb.

This extreme level of darkness is necessary because your retinas do not have pain receptors to warn you of damage. While you might use different optical techniques when spotting the ISS in the evening twilight, solar viewing demands total, uncompromised blockage of light. If a flashlight or a phone screen is easily visible through the lenses, do not use them to look at the sun.

Verifying the safety standard

Look closely at the printed text on the inside arms of the glasses. Safe viewers must be labeled with the ISO 12312-2 international safety standard. However, because some counterfeit manufacturers simply print this label onto substandard cardboard, you cannot rely on the text alone. You should buy only from trusted distributors recommended by reputable astronomical organizations.

It is a common mistake to think that regular sunglasses stacked together can offer the same protection. Standard sunglasses, even those with polarization, transmit thousands of times too much solar energy. Your eyes rely on their natural resolving power to perceive fine details in the landscape, and exposing them to direct sunlight without proper filtration can destroy that delicate ability in less than two seconds.

Storage and future use

If your glasses pass these initial checks, keep them protected from dust and moisture. Store them inside a stiff cardboard envelope or between the pages of a heavy book to prevent creasing. Some manufacturers state that their polymer lenses can be kept indefinitely, while older paper-wrapped models should be discarded after three years.

When the big day arrives, ensure you put the glasses on while looking down, and only then turn your gaze toward the sky. The atmosphere will play its part, and sometimes poor astronomical seeing will make the sun look slightly blurry … but a safe filter ensures you can enjoy the partial phases without risk. Unlike observing Jupiter’s moons on a crisp autumn night, looking at the sun allows absolutely zero margin for error.