Solar eclipse glasses have been created for the sole purpose of protecting your retinas should you be lucky enough to experience a solar eclipse. Solar eclipse glasses are specially designed with a so-called solar filter, an elaborate piece of technology that is engineered to block, among others, the ‘harmful wavelength’ of sun radiation. This solar filter blocks light from outside the visible spectrum (polychromatic ultraviolet or UV and polychromatic infrared or IR radiation) perfectly fine, as well as all intense visible light that might escape quantitative absorbance.

Photo by Jason Howell on Unsplash; Typical eclipse glasses distributed by NASA.

What are they made of?

These glasses are constructed differently from regular sunglasses or tinted glasses. While normal sunglasses usually allow between 10 and 20 percent of daylight (specifically, visible spectrum) to pass through their glasses, the sunlight transmitted can still be too bright for safe viewing of the sun. I’ll write a separate post about ‘‘Understanding How Regular Sunglasses Reduce Visible Light Intensity’’.

Me trying to see the 2017 Great American Eclipse with RayBan. Photo Credit: Alamy.com.

Rick Fienberg, the press officer for the American Astronomical Society (AAS), explained to Space.com that the filters specifically designed for viewing the sun are significantly darker, allowing only a minuscule fraction of light to pass through. In fact, these specialized solar filters are typically around 100,000 times (five order of magnitude) darker than regular sunglasses. He also noted that the majority of ISO-approved eclipse glasses rely on solar filters produced by AstroSolar and Thousand Oaks Optical.

Typically, eclipse glasses are made of a thin metallic film with high optical density. Optical density of film material is an important parameter that ensures the lenses are capable of providing adequate protection against the bright solar corona during the eclipse. A high optical density indicates better protection against UV and IR radiation, and visible light. Hence, these materials make it safer for viewing solar disk during events like eclipses. For example, AstroSolar consists of Baader AstroSolar Silver/Gold Film, which provides eye-sided reflex-free viewing. Additionally, gold blocks infrared (IR) radiation, ensuring there is no unpleasant feeling of warmth in the eyeball. The optical density (OD) of these glasses is OD=5.0, indicating their high level of protection.

Illustration of optical density (OD). Image Credit: [Dr.-Ing. Oliver Pust]. Check his X-post for details.

A short note of on optical density (OD).

OD quantifies a material’s ability to attenuate the intensity of light passing through it. A material with higher OD can absorb light significantly in comparison to a material with lower OD. For example, a material with an OD=2.0 reduces the intensity of light by a factor of 100 in comparison to a material with OD=0.

Is there any alternative to these metallic film glasses?

Yes, there are alternatives to metallic film eclipse glasses. People use the following items e.g., ‘Welder’s Goggles’, ‘Pinhole Camera’, and ‘X-ray films’. X-ray films are most cheapest and creative way to see eclipse/sun. A tweet from the Bureau of Land Management in Arizona suggested a ‘creative way to view the eclipse,’ with the photo of an employee using what appears to be an X-ray to look up at the sun.