Many people suffer serious, incurable eye damage when viewing solar eclipses. To prevent eye damage, you must be informed of the dangers, the many misunderstood and dangerous viewing methods, and the few (but cheap and easy) safe viewing methods.
Looking directly at the sun for more than a second at any time (eclipse or no eclipse) can damage your eyes. Viewing a partial (annular) eclipse and/or the partial phases of a total solar eclipse with unprotected eyes is worse because you are concentrating on looking at the sun, and you look at it longer. It will cause serious injury to your eyes that will affect your vision for the rest of your life. Parts of your retina may literally be cooked!
You will feel nothing (there are no pain receptors in your retinas.) The sunlight may not even be very bright. You will notice no difference for several hours. But then you will notice a difference, and your eyes will be damaged for as long as you live.
Don’t take a chance!
Safe Viewing Methods
There are only three safe viewing methods:
1. Viewing through a special aluminized mylar film designed specifically for eclipse viewing (examples). (The mylar may be made into very inexpensive paper-framed viewers or glasses and sold cheaply, or distributed free.)
2. Viewing through very dark welder’s glass (No. 14)
3. Viewing by projection: you use a card with a small hole in it to project an image of the eclipse on a screen, and you watch the screen, not the sun. You can use a piece of paper, or even a straw hat with a little hole in it. Just put something with a little hole in it over a flat surface and you’ll see the sun’s movement.
Unsafe Viewing Methods
These viewing methods, some hallowed by folk tradition or flawed logic, are unsafe, and you will damage your eyes by using them:
— Sunglasses, single or multiple layers
— Smoked glass
— All color film, black-and-white film that contains no silver, photographic negatives with images on them (x-rays and snapshots)
— Polarizing filters
— Photographic neutral density filters
— Mylar film not of optical grade (such as product packaging)
The Good News
If you are in the path of totality, and during the totality of the eclipse, it is safe (and recommended!) to view the sun with the naked eye. But before and after totality, or if you are not within the path of totality, you should use the safe viewing methods mentioned above.
Here is more information, adapted from an article entitled “Eye Safety During Solar Eclipses” by B. Ralph Chou, MSc, OD, Associate Professor, School of Optometry, University of Waterloo, Adapted from NASA RP 1383 Total Solar Eclipse of 1999 August 11, April 1997, p. 19.
Observing the Sun can be dangerous if you do not take the proper precautions. The solar radiation that reaches the surface of Earth ranges from ultraviolet (UV) radiation at wavelengths longer than 290 nm to radio waves in the meter range. The tissues in the eye transmit a substantial part of the radiation between 380 and 1400 nm to the light-sensitive retina at the back of the eye. While environmental exposure to UV radiation is known to contribute to the accelerated aging of the outer layers of the eye and the development of cataracts, the concern over improper viewing of the Sun during an eclipse is for the development of “eclipse blindness” or retinal burns.
Exposure of the retina to intense visible light causes damage to its light-sensitive rod and cone cells. The light triggers a series of complex chemical reactions within the cells which damages their ability to respond to a visual stimulus, and in extreme cases, can destroy them. The result is a loss of visual function which may be either temporary or permanent, depending on the severity of the damage. When a person looks repeatedly or for a long time at the Sun without proper protection for the eyes, this photochemical retinal damage may be accompanied by a thermal injury – the high level of visible and near-infrared radiation causes heating that literally cooks the exposed tissue. This thermal injury or photocoagulation destroys the rods and cones, creating a small blind area. The danger to vision is significant because photic retinal injuries occur without any feeling of pain (there are no pain receptors in the retina), and the visual effects do not occur for at least several hours after the damage is done [Pitts, 1993].
The only time that the Sun can be viewed safely with the naked eye is during a total eclipse, when the Moon completely covers the disk of the Sun. It is never safe to look at a partial or annular eclipse, or the partial phases of a total solar eclipse, without the proper equipment and techniques. Even when 99% of the Sun’s surface (the photosphere) is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause a retinal burn, even though illumination levels are comparable to twilight [Chou, 1981, 1996; Marsh, 1982]. Failure to use proper observing methods may result in permanent eye damage or severe visual loss. This can have important adverse effects on career choices and earning potential, since it has been shown that most individuals who sustain eclipse-related eye injuries are children and young adults [Penner and McNair, 1966; Chou and Krailo, 1981].
The same techniques for observing the Sun outside of eclipses are used to view and photograph annular solar eclipses and the partly eclipsed Sun [Sherrod, 1981; Pasachoff & Menzel 1992; Pasachoff & Covington, 1993; Reynolds & Sweetsir, 1995]. The safest and most inexpensive method is by projection.A pinhole or small opening is used to form an image of the Sun on a screen placed about a meter behind the opening. Multiple openings in perfboard, in a loosely woven straw hat, or even between interlaced fingers can be used to cast a pattern of solar images on a screen. A similar effect is seen on the ground below a broad-leafed tree: the many “pinholes” formed by overlapping leaves creates hundreds of crescent-shaped images.
Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the Sun onto a white card. All of these methods can be used to provide a safe view of the partial phases of an eclipse to a group of observers, but care must be taken to ensure that no one looks through the device. The main advantage of the projection methods is that nobody is looking directly at the Sun.The disadvantage of the pinhole method is that the screen must be placed at least a meter behind the opening to get a solar image that is large enough to see easily.
The Sun can only be viewed directly when filters specially designed to protect the eyes are used. Most such filters have a thin layer of chromium alloy or aluminum deposited on their surfaces that attenuates both visible and near-infrared radiation. A safe solar filter should transmit less than 0.003% (density~4.5) of visible light (380 to 780 nm) and no more than 0.5% (density~2.3) of the near-infrared radiation (780 to 1400 nm).
One of the most widely available filters for safe solar viewing is shade number 14 welder’s glass, which can be obtained from welding supply outlets. A popular inexpensive alternative is aluminized mylar manufactured specifically for solar observation. (“Space blankets” and aluminized mylar used in gardening are not suitable for this purpose!) Unlike the welding glass, mylar can be cut to fit any viewing device, and doesn’t break when dropped. Many experienced solar observers use one or two layers of black-and-white film that has been fully exposed to light and developed to maximum density. The metallic silver contained in the film emulsion is the protective filter. Some of the newer black and white films use dyes instead of silver and these are unsafe. Black-and-white negatives with images on it (e.g., medical x-rays) are also not suitable. More recently, solar observers have used floppy disks and compact disks (both CDs and CD-ROMs) as protective filters by covering the central openings and looking through the disk media. However, the optical quality of the solar image formed by a floppy disk or CD is relatively poor compared to mylar or welder’s glass. Some CDs are made with very thin aluminum coatings which are not safe – if you can see through the CD in normal room lighting, don’t use it!! No filter should be used with an optical device (e.g. binoculars, telescope, camera) unless it has been specifically designed for that purpose and is mounted at the front end (i.e., end towards the Sun). Some sources of solar filters are listed in the following section.
Unsafe filters include all color film, black-and-white film that contains no silver, photographic negatives with images on them (x-rays and snapshots), smoked glass, sunglasses (single or multiple pairs), photographic neutral density filters and polarizing filters. Most of these transmit high levels of invisible infrared radiation which can cause a thermal retinal burn (see Figure 24). The fact that the Sun appears dim, or that you feel no discomfort when looking at the Sun through the filter, is no guarantee that your eyes are safe. Solar filters designed to thread into eyepieces that are often provided with inexpensive telescopes are also unsafe. These glass filters can crack unexpectedly from overheating when the telescope is pointed at the Sun, and retinal damage can occur faster than the observer can move the eye from the eyepiece.
In spite of these precautions, the total phase of an eclipse can and should be viewed without any filters whatsoever. The naked eye view of totality is not only completely safe, it is truly and overwhelmingly awe-inspiring!