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July 2009
Eclipse Blindness: Myth or Reality?
Dr Sandeep Arora

Total solar eclipse is probably the most spectacular astronomical event that most people will experience in their lives. There is a great deal of interest in watching eclipses, and thousands of astronomers travel around the world to observe and photograph them.

But many people do not utilize the chance and never enjoy the free cosmic thrill of a total solar eclipse because they believe the myth that even being outdoors may make them go blind.

Don't use these as solar filters
  • Sunglasses
  • Photographic neutral density filters
  • Smoked glass
  • Polarizing filters
  • Compact discs
  • Floppy disk media
  • Black colour film
  • Any black and white film negatives bearing images
So how true is the myth?
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 lightsensitive 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.

The first thing to realize is that as the sun slides into eclipse. It does not begin to emit new and strange forms of damaging radiation - it just squirts out what it always has.

What this means is that you can certainly damage your eyes by staring at the sun when it is partially covered by the moon - in exactly the same way as you can damage your eyes by looking at the full sun. The second thing to understand is the mechanism of how the sun can damage your eyes. The sun gives out light, but it also gives out heat energy. This heat energy is focused and concentrated onto the central part of your retina, which deals with fine vision (The lens of your eye is no different to a magnifying glass that bundles the sun's rays and sets a piece of paper alight). If you stare at the sun for long enough, you will burn out that central part of the retina. Your peripheral vision will be fine, so you'll still notice something approaching from the corner of your eye. But if you try to read any fine print, it'll be fuzzy, as if you were looking through seawater, or a glass smeared with petroleum jelly.

How to Watch a Solar Eclipse
Never look at the sun directly – doing so can damage your eyes. The best way to observe the sun is by projecting the image.

The Pinhole Projection Method:
Here is one way to project the sun's image:
  1. Get two pieces of cardboard (flaps from a box, backs of paper tablets).
  2. With a pin or pencil point, poke a small hole in the center of one piece (no bigger than the pin or pencil point).
  3. Take both pieces in your hand.
  4. Stand with your back to the sun.
  5. In one hand, hold the piece with the pinhole; place the other piece (the screen) behind it.
  6. The sunlight will pass through the pinhole and form an image on the screen.
  7. Adjust the distance between the two pieces to focus and change the size of the image.
Symptoms of eclipse burns
  • Temporary or permanent loss of central or peripheral vision
  • Distorted or blurred vision
  • Dark patches in the field of vision
  • Unusual colors
  • Light sensitivity
  • Sore eyes

Do not look through the pinhole at the sun. Never ever look at the sun, especially the partial phase through unprotected eyes or homebrew filtering mechanisms.

Solar Filters
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.

One such type of filter is made of aluminized polyester. Another 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.

Welders' Goggles
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. Warning! Do not attempt to use these filters behind a pair of binoculars or telescope (that is, between your eyes and the 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.

Camera and Telescope Solar Filters
Telescope and camera companies provide metal-coated filters that are safe for viewing the sun. They are more expensive than common Mylar.

Fully Exposed and Developed Black-and-White Film
You can make your own filter out of black-and-white film, but only true black-and-white film (such as Kodak Tri-X or Pan-X). Such films have a layer of silver within them after they are developed. It is this layer of silver that protects your eyes.

To make your own solar filter, proceed as follows. Open up a roll of black-andwhite 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. With this filter, you can look directly at the sun with safety.
Dr Sandeep Arora is Senior Consultant Ophthalmologist, and Corneal & Refractive Surgeon at Apollo Hospitals, Ahmedabad.

    
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