Many of you have obtained solar eclipse viewing glasses, but are they really safe to view the solar eclipse directly? If so, how long can one safely look at the eclipse before risking eye damage? The absolute safest way to view the eclipse is indirectly (pinhole viewing) with or without the eclipse glasses, but what about looking at the sun with them on? What is the difference between ISO certified glasses and non-ISO certified glasses? What part of sunlight is dangerous to the eye and do the eclipse glasses block it? Regardless of what’s stamped on the temple how do you know if your glasses are really ISO certified? What about welders glasses?
While I don’t have all the answers, it’s important to know that the ISO standard is in place to ensure that there is energy being blocked from the eye in the infrared and not just visible and UV parts of the spectrum. Infrared light is the wavelengths of light the sun emits that are damaging to the light-sensing tissue on the back of the eye, called the retina. Most people think black glass is safe to use to view the sun, but even black glass can pass dangerous light from the infrared. What’s risky is that the infrared may be so dim or not at all visible yet still transmit at levels that are damaging to the eye. If you spend too much time looking at the sun through glasses that don’t fully protect from the infrared (10 seconds or more generally) damage is likely to occur.
Welders glasses aren’t safe because generally, welding doesn’t expose welders to infrared light, so they are not designed to block infrared, only UV and visible light. Generally, if a filter is not ISO certified it should be assumed to be transparent in the infrared. However, if the filter has a mirror-like appearance the metal coating DOES protect in the infrared for the most part. The problem with metal coatings is the occurrence of pinholes that can let damaging levels through. Pinholes, however, are easy to notice.
In summary, you should not view the eclipse through any kind of eclipse viewing glasses, ISO certified or not for any period of time. If the glasses have a mirror coating I still don’t recommend you look directly at the sun but if you do anyway be certain minimize the viewing time below 5 seconds…probably WELL below 5 seconds to be on the safe side.
Thanks to Optical Scientist Robert Winsor for his help with this information.