By Anastacio Dalde III, U.S. Army Public Health Center, Aberdeen Proving Ground, MarylandAugust 1, 2017
FORT RUCKER, Ala. (August 1, 2017) - On Aug. 21, 2017, North America will have the opportunity to view a total solar eclipse. It will be the first total eclipse in the continental United States in nearly 40 years, and the first coast-to-coast eclipse in a century. The eclipse will make landfall on the west coast at 10:15 a.m. (PDT) just north of Newport, Oregon. Traveling at more than 1,600 mph, the shadow will move across the country in just over an hour and a half before leaving south of McClellanville, South Carolina, at 2:49 p.m. (EDT).
A solar eclipse occurs when the moon passes between Earth and the sun, obscuring the sun - either partially or totally - from a viewer on Earth. Most people in North America will be able to view at least a partial eclipse, while those in some states will see a total solar eclipse. Figure 1 shows the eclipse's roughly 70-mile-wide path through the U.S. as the shadow travels from west to east.
Viewing a total solar eclipse is a unique and worthwhile experience; but if not done correctly, eye injuries can occur. Most of us would never stare directly at the sun because we know it can cause permanent eye damage. During an eclipse, though, the lower light levels may tempt some to watch it without suitable eye protection. This, too, is extremely hazardous. Figure 2 shows a retinal lesion caused by staring at the sun without proper eye protection. While most people gradually recover their normal vision within one to six months, some end up with permanent blurry vision and central blind spots.
Indirect viewing using the pinhole-projection method, illustrated in Figure 3, will be the safest way to enjoy this eclipse. NASA has put together an excellent resource showing how to make a pinhole camera using only cardstock, aluminum foil, tape and a paper clip or pin at https://www.jpl.nasa.gov/edu/learn/project/how-to-make-a-pinhole-camera/. This simple tool will allow eclipse viewers to experience the event without risking damaging their eyesight.
For those who still want to view the eclipse directly, special eyewear is needed to avoid injury. Eyewear, including eclipse, prescription and safety glasses and regular sunglasses, are typically certified to various national and international specifications. For example, the American National Standards Institute publishes consensus standards with specifications for prescription eyeglasses, safety glasses used for impact or chemical protection, and sunglasses. The European economic area requires CE (European Conformity) certification for eyewear. The International Organization for Standardization also provides certification of eyewear. Only ISO 12312 2 specifically addresses the safety of solar eclipse eyewear for direct viewing.
While many manufacturers claim their eclipse glasses are specifically made for safe viewing, our informal laboratory tests suggest that not all eyewear offers sufficient protection. Consider the following factors when purchasing solar eclipse eyewear:
• The best eclipse eyewear has the ISO 12312-2 certification (see Figure 4). Many of the solar eclipse eyewear manufactured in the United States meet this ISO standard. We tested 25 samples of eclipse eyewear and found the ISO-certified glasses consistently provided adequate protection required to view the sun during an eclipse. All of the ISO-certified eclipse eyewear had additional CE certification markings.
• Eclipse eyewear with only CE certification markings might not offer sufficient protection. We tested two samples of eclipse eyewear that had CE certification markings, but no ISO certification markings. The first sample, manufactured in China, did not provide sufficient protection in our laboratory tests (see Figure 5) when compared to the ISO 12312-2 standard. The second sample, manufactured in the United Kingdom, provided so much protection that it may be too dark for viewing the eclipse (see Figure 6).
• Some types of welding glass also offer sufficient protection for viewing an eclipse safely. Welding glass comes in different shade numbers which characterize its level of protection. The higher the shade number, the darker the lens and more protection provided at visible wavelengths. Use at least Shade 14 welding glasses to view the eclipse. Shade 5 welding glasses are commonly marketed as eclipse glasses, but these do not provide enough protection.
• Sunglasses and safety glasses used for everyday sun protection and for occupational safety eye protection (including Military Combat Eye Protection sunglasses) do not provide the minimum protection to directly view the eclipse (see Figure 7). Nevertheless, some safety glasses are marketed with the word "eclipse" in the name. Others advertise that the eyewear "Meets and Exceeds ANSI Z87." However, ANSI Z87 has no safety specifications for direct viewing of the sun. ANSI Z87 is a standard for occupational safety glasses for protection against impact, dust, chemical splash and welding.
• Avoid various do-it-yourself techniques for making your own eclipse eyewear, which can be found in instructional videos/websites on the Internet.
If you suspect you have experienced an eye injury due to viewing the eclipse, get an evaluation by an eye care professional as soon as possible. Symptoms might develop immediately or in a few days. The severity or type of symptoms may also change over time. The most common indications of possible injury are blurry vision and central blind spots. Color vision can also be affected.
In closing, we hope many people will be able to experience the upcoming solar eclipse. To view the eclipse safely, remember to use eclipse eyewear with ISO 12312-2 certification for direct viewing or the pinhole-projection method for indirect viewing. While it will be a magnificent event, it's not worth risking your eyesight.