Here is information directly from the NASA site:
"Partial eclipses, annular eclipses, and the partial phases of total eclipses are never safe to watch without taking special precautions. Even when 99 percent of the sun's surface is obscured during the partial phases of a total eclipse, the remaining photospheric crescent is intensely bright and cannot be viewed safely without eye protection. Do not attempt to observe the partial or annular phases of any eclipse with the naked eye. Failure to use appropriate filtration may result in permanent eye damage or blindness."
The site reports that the safest ways to see an eclipse is by 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 leafy tree."
NASA also says binoculars may 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."
The site also indicates that the sun can be viewed directly only when using filters specifically designed for this purpose. Such filters usually have a thin layer of aluminum, chromium or silver deposited on their surfaces that attenuates ultraviolet, visible, and infrared energy. "One of the most widely available filters for safe solar viewing is a number 14 welder's glass, available through welding supply outlets."
Another option it suggests is aluminized Mylar which can easily be cut with scissors and adapted to any kind of box or viewing device. "No filter is safe to use with any optical device (i.e. - telescope, binoculars, etc.) unless it has been specifically designed for that purpose."
Filters that you want to stay clear of and that NASA considers unsafe include: color film, some non-silver black and white film, medical x-ray films with images on them, smoked glass, photographic neutral density filters and polarizing filters, the site reports.
In addition, NASA warns not to experiment with other filters unless you are certain that they are safe. "Damage to the eyes comes predominantly from invisible infrared wavelengths."
Bottom-line the site reports: "Even if the sun appears dark in a filter or if you don't feel any discomfort, does not guarantee that your eyes are safe. Avoid all unnecessary risks."
"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 completely safe and is overwhelmingly awe-inspiring," the site reports.
