The Schepens Eye Research Institute
There is no danger to the eye in looking directly at a total solar eclipse. However; looking directly at the smallest part of a partial eclipse, including any annular eclipse, is very dangerous and can result in retinal damage. Therefore, one must be careful just before totality and particularly as the total phase ends when watching this most wonderful phenomenon. Just after the first bright spot, the photosphere, reemerges from behind the moon, it is time to look away.
Whenever a solar eclipse is expected there is much uninformed fright about watching the event. It is correct to be especially cautious in advising children, as young eyes are most at risk during the partial eclipse, their lens and media is most clear, and the temptation to stare great.
Danger to the eye is from heat ( infrared radiation ), UV ( ultraviolet radiation ), and from excessive blue light. The heat risk is perhaps the best understood, since we are familiar with using a lens to focus the sun to burn paper. A momentary glance, such as we do occasionally on a sunny day, does indeed focus a very intense image of the sun on the transparent neural tissue at the back of the eye, the retina, and a highly absorbent layer just beyond, but we have a reflex to avert our eye and the heat buildup is brief and little damage results. If one closes the eye after such a chance event, one will note a series of bright 'after images' of the sun staggered irregularly, indicating the eye's protective motion during the glance. Since there is nothing novel to see, a simple disk, no specific fixation results. UV radiation can cause 'sunburn' to the cornea or outer surface of the eye, just like sunburn to the skin - same mechanism, similar damage, but for the eye it results in pain and vision loss. The retina is at risk from a very small part of the UV that is transmitted through the ocular media and lens. This risk is greatest for young eyes, and in general adults beyond 30 years of age have enough yellow in the lens and absorption in the media that UV after atmospheric absorption is less of a problem than the heat. The least well understood risk is from blue light that seems implicated in biochemical damage to receptor cells and their environment in the sensitive neural tissue. Documentation is growing that excessive exposure to blue light may, in most individuals, result in Macular Degeneration and blindness in people when they become older.
Again there is all this caution about viewing a solar eclipse because there are serious risks during the partial phases. The radiation to the eye during the total, or dark part of the eclipse, is less than when viewing the full moon, and that is safe for viewing for many minutes, and a total solar eclipse rarely lasts seven minutes. I am always asked why the partial eclipse is so special a hazard, since most days we can safely glance at the sun without care. The danger comes from the fact an eclipse changes the circular, too bright disk, into something interesting. The brightness is the same and still too great, but in eclipse there is a shape, a black part, and the crescent remaining of the sun. Now the normal safety feature of eye motion is defeated by the cognitive event of having a point to fixate. The two sharp cusps are points that the eye may focus upon and now the damaging image on the retina is stopped on the most sensitive neural tissue. This short stoppage begins damage from all mechanisms, too much blue light, too much UV, and too much heat. This damage is not recoverable.
The exact duration that will cause loss of sight in that area of the retina varies greatly from eye to eye. Again greatest risk is to the young eye, and least to the oldest eyes that absorb and scatter the damaging radiation.
To observe these bright partial phases of the eclipse a projected image is safest. There are many filter options, but great caution must be exercised in selection and use.
NO damage or risk to the eye is involved when looking at the total solar eclipse, even with optical aids, such as binoculars. BUT the bright disk, no matter how small or partial crescent, will cause damage, and optical aids must not be used unless special filters are used and informed supervision is on hand.
It is my greatest disappointment that so many have missed seeing one of nature’s most beautiful events - a total solar eclipse - because of misinformation. No photograph, no TV or other laboratory technique can represent or capture this unique physical phenomenon. The colors and contrast, the detail and structure of the image is beyond reproduction.
Even if this were not so there is a primitive human response to seeing the sun go dark, and blood red dots appear around the black, unlit, side of the moon, and brilliant silver green hair like streamers reaching outward on either side, the object seems to fill the sky. The many visual phenomena that come into play making a total solar eclipse so wonderful an event should not be missed.