Staying safe during the eclipse

Thy beams, so reverend and strong—

Why shouldst thou think?

I could eclipse and cloud them with a wink

John Donne, The Sun Rising, c1600.

  On 11 August at 11 11 am, a total solar eclipse will occur over south west England and north west France. Its path, over 100 miles wide, leads it east and south across the continent of Europe, passing from the north of France to Munich, and thence to Bucharest and the Middle East. A partial eclipse may be visible from the north of Scotland to the north of Spain and from northern Poland to southern Italy. The eclipse will occur at the height of summer, as the sun nears its zenith, over some of the most densely populated regions of the world, and thus may be followed by an even greater incidence of retinal injury than reported after other recent eclipses.^1–5

There seems to be two mechanisms of retinal injury from solar radiation. Viewing the sun through binoculars or telescopes produces the 10-25° temperature rise in the retina required for a thermal burn. By contrast, looking at the sun with the naked eye induces photochemical injury to retinal receptor cells and pigment epithelium, associated with only a 4° rise in retinal temperature.⁶ This second process represents the common form of solar retinopathy, as reported in aircraft lookouts, malingerers, sunbathers, the mentally ill, users of lysergic acid, and pilgrims, as well as those watching a partial or total solar eclipse.

A range of individual and environmental factors may influence susceptibility to solar retinopathy, including pupil size, refractive state, dark adaptation, pigmentation, body temperature, nutritional state, zenith angle of the sun, atmospheric conditions, and the thickness of the ozone layer. Fractionated exposure may be more damaging than continuous, and short wavelengths more than long. Damage seems to be cumulative.⁶ It is possible to identify groups at particular risk. These include children and teenagers, because the lens of the eye filters little short wavelength light before the age of 20 years; people with ocular conditions such as retinal dystrophies or albinism or who have undergone certain forms of cataract surgery; those taking photosensitising medication; and those using alcohol or recreational drugs. However, considerable variation in susceptibility to solar retinopathy is unexplained, some individuals seeming to be relatively resistant and others developing symptoms after as little as 30-60 seconds’ exposure.⁷

No pain is felt at the time of injury. Symptoms take some days to develop and include reduction in visual acuity, central scotomas, impairment of colour vision, and distortion of straight lines. Retinal changes consist initially of a yellow retinal spot surrounded by a faint grey halo, followed by a reddish spot, sometimes with a pigment halo, and finally a sharply circumscribed defect in the inner retina. Histopathological studies show damage at the apices of retinal pigment epithelial cells and to photoreceptor cells. In some eyes these changes seem to be reversible; in others, there is permanent loss of photoreceptors.

Typically some spontaneous resolution occurs over subsequent weeks, its extent mirroring the severity of the initial injury. In the largest series, half the eyes recovered 6/6 visual acuity, and about 10% were left with visual acuity less than 6/12, and persistent scotomata.¹ This central visual impairment does not equate to blindness but may restrict choice of occupation in younger patients. Furthermore, because photochemical damage is cumulative, there are concerns that such injury may predispose to later disease. Some epidemiological evidence links advanced age related macular degeneration to exposure to short wavelength light.⁸

No treatment has been shown to be effective in solar retinopathy. The emphasis is therefore on prevention, and in particular, children must be closely supervised. The only safe time to look at the sun is during the short two minute period of total eclipse, and one should look away the moment the first rays of the sun appear at the edge of the moon. It is unsafe to look at the sun during the partial phases of a total eclipse, or during a partial eclipse. Binoculars and telescopes should not be used. Smoked glass, photographic film, compact discs, one or several pairs of sunglasses, and sun caps for optical instruments do not afford adequate protection. Solar viewers, often made of aluminised polyester, are widely available, and those carrying the CE mark have been tested to comply with European Union personal protection equipment standards. However, concerns have been expressed about such viewers because a single size viewer is unlikely to fit all children’s faces accurately, because surface damage may compromise any protective effect, and because the importance of physical defects in the aluminium film remains uncertain.

For these reasons, the Department of Health, the Royal College of Ophthalmologists, and the College of Optometrists advise against any direct viewing of the sun; they say that the safest way to view the eclipse is by projection. A simple way of doing this is to turn one’s back to the sun and use a 2 mm pinhole cut in a card to project the image of the sun on to a second card a metre away. This image can be viewed safely.

Solar retinopathy may occur rapidly, without pain, and without being immediately apparent. Children are commonly affected, there is no treatment, and permanent visual loss may result. Indirect viewing using projection offers the safest means of witnessing the eclipse.