The Sun can be viewed safely with the naked eye only during the few brief minutes of a total solar eclipse. Annular and partial solar eclipses are never safe to watch without taking special precautions. Although more than 88% of the Sun's surface is obscured during May's annular phase, the remaining photospheric annulus is intensely bright and cannot be viewed directly without eye protection. Do not attempt to observe the partial or annular phases of the eclipse with the naked eye. Failure to use appropriate filtration may result in permanent eye damage or blindness!
Generally, the same equipment, techniques and precautions used to observe the Sun outside of eclipse are required [Chou, 1981; Marsh, 1982]. There are several safe methods which may be used to watch the partial and annular phases. The safest of these is projection, in which a pinhole or small opening is used to cast the image of the sun on a screen placed a half-meter or more beyond the opening. Projected images of the sun may even be seen on the ground in the small openings created by interlacing fingers, or in the dappled sunlight beneath a tree. Binoculars can also be used to project a magnified image of the sun on a white card, but you must avoid the temptation of using these instruments for direct viewing.
Direct viewing of the sun should only be done using filters specifically designed for this purpose. Such filters usually have a thin layer of aluminum, chromium or silver deposited on their surfaces which attenuates both the visible and the infrared energy. Experienced amateur and professional astronomers may use one or two layers of completely exposed and fully developed black-and-white film, provided the film contains a silver emulsion. Since developed color films lack silver, they are unsafe for use in solar viewing. A widely available alternative for safe eclipse viewing is a number 14 welder's glass. However, only mylar or glass filters specifically designed for the purpose should used with telescopes or binoculars.
Unsafe filters include color film, smoked glass, photographic neutral density filters and polarizing filters. Deep green or grey filters often sold with inexpensive telescopes are also dangerous. They should not be used for viewing the sun at any time since they often crack from overheating. Do not experiment with other filters unless you are certain that they are safe. Damage to the eyes comes predominantly from invisible infrared wavelengths. The fact that the sun appears dark in a filter or that you feel no discomfort does not guarantee that your eyes are safe. Avoid all unnecessary risks. Your local planetarium or amateur astronomy club is a good source for additional information.
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