A solar eclipse can be viewed safely with the naked eye only during the few brief seconds or minutes of a total solar eclipse, when the Sun itself is completely obscured by the Moon. To make your own solar filter, proceed as follows. Open up a roll of black-and-white film and expose it to the Sun for a minute. Have it developed to provide you with negatives. Use the negatives for your filter. It is best to use two layers.
Photo: Kenway.Lee
With this filter, you can look directly at the Sun with safety. Remember, however, that if you are planning to use black-and-white film as a solar filter, you need to prepare it at least several days in advance. Standard or polaroid sunglasses are not solar filters. They may afford some eye relief if you are outside on a bright day, but you would never think of using them to stare at the Sun. So you cannot use sunglasses, even crossed polaroids, to stare at the Sun during the partial phases of an eclipse. They provide little or no eye protection for this purpose.
The first thing to remember about observing an eclipse is safety. A lunar eclipse an eclipse of the Moon is perfectly safe to watch with the naked eye; you’re only looking at the Moon, at night, which is quite safe. A solar eclipse is potentially dangerous.
However, because viewing a solar eclipse involves looking at the Sun, which can damage your eyesight. Welders’ goggles or the filters for welder’s goggles with a rating of 14 or higher are safe to use for looking directly at the Sun. They are also relatively inexpensive. Do not attempt to use these filters behind a pair of binoculars or telescope. The magnifying optics of these devices will focus the full power of the Sun onto the welder’s filter which could crack and shatter from the intense heat after only a few minutes. If you wish to observe the eclipse with binoculars or a telescope, you must use a specially designed solar filter on the front end of the instrument. These filters are discussed in the next section.
Photo: igorfp
A second technique for viewing the Sun safely is by looking at it directly through a specially designed solar filter. Such filters permit only a miniscule fraction of the Sun’s light to pass through them. Advertisements for solar filters may be found in popular astronomy magazines.
One such type of filter is made of aluminized polyester. Beware, though, that polyester, a plastic, comes in various thicknesses and with various coatings. You need a metal coating to save your eyesight and you need to examine the Polyester for small holes that could allow unfiltered sunlight to reach your eyes and damage them. A good solar filter will allow you to look comfortably at the filament of a high-intensity electric lamp.
It is never safe to look directly at the Sun except during a total eclipse; a partial or annular eclipse, even when the Sun is mostly covered, can still cause permanent eye damage, even though you might not feel any discomfort. Looking at the Sun through any kind of optical aid is extremely dangerous, and can cause permanent blindness.
Photo: Manuel Secher
Sunglasses do not provide anything like adequate protection, as they do not block the wavelengths of light which are likely to damage your eyes, or reduce the intensity of the visible light sufficiently. Various other ad-hoc solar filters, such as welder’s goggles or using fully exposed and developed black-and-white negatives, are sometimes discussed, but unless you know exactly what you are doing, can be extremely dangerous, and so can’t be recommended.
A second type of solar filter is made from a black polymer which gives a yellow/orange tint to the Sun which is more pleasing than the bluish color seen with aluminized polyester filters. Either filter type is completely safe provided that it has an optical density of 5.0 or more. This means that only 0.01% of the Sun’s light can pass through the filter.
When using any kind of filter, however, do not stare for long periods at the Sun. Look through the filter briefly and then look away. In this way, a tiny hole that you miss is not likely to cause you any harm. You know from your ignorant childhood days that it is possible to glance at the Sun and immediately look away without damaging your eyes. Just remember that your eyes can be damaged without you feeling any pain.
Sources: hermit.org, mreclipse.com
