Do you need specialist filters to look at the sun?

Question

During the UK's recent solar eclipse we were constantly reminded that we should never look directly at the sun because it could damage your eyesight permanently, that you should only look at the eclipse through specialist filters, with a pin-hole camera, and so on. One outlet even recommended using a kitchen colander!

But I also remember Richard Feynman writing in his book (Surely You're Joking, Mr. Feynman) that he didn't bother with the heavily darkened goggles they had given everyone who was present at the Trinity test to observe the detonation. He wrote that the only component of the blast that could physically damage your eyes was the UV emissions, and that an ordinary car windscreen was perfectly capable of blocking all the potentially harmful wavelengths.

They gave out dark glasses that you could watch it with. Dark glasses! Twenty miles away, you couldn't see a damn thing through dark glasses. So I figured the only thing that could really hurt your eyes (bright light can never hurt your eyes) is ultraviolet light. I get behind a truck windshield, because the ultraviolet can't go through glass, so it would be safe, and I could see the damn thing.

Isn't it true that it's the UV emissions of the sun that also make it dangerous to look at? Therefore, wouldn't you be safe using an ordinary sheet of glass to observe the sun? Or are there other things the sun emits that can also damage eyesight and which aren't blocked by ordinary glass?

Answer 1

This paper, for example, lists cases of solar retinopathy which suffered by people who were using glass (smoked glass, stained glass, sunglasses, etc.):

Brit. J. Ophthal. (I969) 53, 534
Visual prognosis after solar retinopathy
P. A. MAcFAUL

Injury is most likely to occur only on determined fixation, and even when this lasts for only a brief period permanent visual damage may result. There is no generally available means of protection against the injurious effect of radiation. The results of this and previous studies show that all the so-called protective devices, such as sun glasses, filters, smoked or stained glass, or old negatives do not give effective protection and may even facilitate injury by prolonging the time of exposure.

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For a second opinion, this paper (from NASA's web site) claims that you need a filter which attenuates visible and infrared light:

Observing the Eclipse
Eye Safety And Solar Eclipses
B. Ralph Chou, MSc, OD
Associate Professor, School of Optometry, University of Waterloo

Exposure of the retina to intense visible light causes damage to its light-sensitive rod and cone cells. The light triggers a series of complex chemical reactions within the cells which damages their ability to respond to a visual stimulus, and in extreme cases, can destroy them. The result is a loss of visual function which may be either temporary or permanent, depending on the severity of the damage. When a person looks repeatedly or for a long time at the Sun without proper protection for the eyes, this photochemical retinal damage may be accompanied by a thermal injury - the high level of visible and near-infrared radiation causes heating that literally cooks the exposed tissue. This thermal injury or photocoagulation destroys the rods and cones, creating a small blind area. The danger to vision is significant because photic retinal injuries occur without any feeling of pain (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 [Pitts, 1993].

The Sun can only be viewed directly when filters specially designed to protect the eyes are used. Most such filters have a thin layer of chromium alloy or aluminum deposited on their surfaces that attenuates both visible and near-infrared radiation. A safe solar filter should transmit less than 0.0003% (density~4.5)10 of visible light (380 to 780 nm) and no more than 0.5% (density~2.3) of the near-infrared radiation (780 to 1400 nm). Figure 24 shows the spectral response for a selection of safe solar filters.

One of the most widely available filters for safe solar viewing is shade number 14 welder's glass, which can be obtained from welding supply outlets. A popular inexpensive alternative is aluminized mylar manufactured specifically for solar observation.