A total eclipse of the Sun is probably the most spectacular astronomical event that people can experience. Every year there are at least two solar eclipses of one kind or another (and sometimes as many as five) visible from different parts of the world.

In the days and weeks before an eclipse occurs, news stories in the media provide information on what will happen and how to watch the event safely. Unfortunately, despite the best intentions, inaccurate or confusing information on safe observing techniques is often provided. This is especially true when the recommendations concern protective filters for directly observing the Sun. (Here is a list of solar filter suppliers.)

I first published solar filter data in Sky & Telescope's August 1981 issue (page 119); in the years since then new filters intended for both visual and photographic uses have come on the market. In June 1996 I participated in a NATO-sponsored meeting on solar-eclipse astronomy. This prompted me to make spectrophotometric measurements of a variety of materials and assess whether they provide adequate protection for the eyes. These included such oddball items as the internal magnetic disk of a 3½-inch floppy, multiple layers of space blanket (a very thin type of aluminized polyester film), CDs, and metal-coated polyester food packaging.

How the Eye is Damaged

Solar radiation reaching the surface of the Earth ranges from ultraviolet light at wavelengths as short as 290 nanometers (2,900 angstroms) to radio waves in the meter range. Lifetime exposure to solar ultraviolet radiation is an established contributor to accelerated aging of the outer layers of the eye and skin and the development of cataracts.* But more immediate damage takes place from directly observing the Sun with inadequate eye protection. The eye will transmit most of the radiation between 380 nanometers (violet) and 1,400 nanometers (near infrared) to the light-sensitive retina, resulting in retinal burns.

Exposing the retina to high-intensity visible light triggers a series of complex chemical reactions within the light-sensitive rod and cone cells. The products of these reactions impair the cells' ability to respond to light and in extreme cases can destroy them. Depending on the severity of the damage, an affected observer experiences either a temporary or permanent loss of visual function. This photochemical injury occurs mainly when the retina is exposed to blue and green light.

When longer wavelengths of visible and near-infrared radiation pass into the eye, they are absorbed by the dark pigment epithelium below the retina. The energy is converted into heat that can literally cook the exposed tissue. Photocoagulation destroys the rods and cones, leaving a permanently blind area in the retina. This thermal damage also occurs during extended exposure to blue and green light.

Both photochemical and thermal retinal injuries occur without the victim's knowledge, as 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.

For wavelengths between 380 and 1,400 nanometers, we find that a filter with a transmittance of 0.0032 percent, corresponding to a shade number of 12, provides "adequate" retinal protection during solar viewing. However, this does not take into account visual comfort, in which case a darker filter having a transmittance of 0.0003 percent (shade number 14) is often preferable.

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* Editor's note: If you wear glasses outdoors, you can greatly reduce your lifetime UV exposure by having UV-blocking coatings applied to your glasses. This option is so cheap and easy that anyone should do it. Also, polycarbonate lenses block UV without needing coatings.