This thermal damage also occurs during extended exposure to blue and green light. Longer wavelengths of visible light and near-IR radiation are absorbed by the dark pigment epithelium below the retina, and converted into heat which can literally cook the exposed tissue. This photochemical retinal injury mainly occurs when the retina is exposed to blue and green light. Depending on the severity of the damage, an affected observer experiences either a temporary or permanent loss of visual function. The resulting photoproducts damage the cells, impairing their ability to respond to light, and in extreme cases can destroy them.
However, observing the sun with inadequate or no eye protection results in "eclipse blindness" or retinal burns because the eye transmits most of the optical radiation between 380 nm and 1400 nm to the light-sensitive retina.Įxposure of the retina to high irradiance levels of visible light triggers a series of complex chemical reactions within the light-sensitive rod and cone cells.
It is widely accepted that environmental exposure to high levels of solar UV radiation contributes to the accelerated ageing of the outer layers of the eye and skin, and the development of cataracts. Solar radiation reaching the surface of the earth ranges from ultraviolet (UV) radiation at wavelengths longer than 290 nm to radio waves in the metre range. There are now several manufacturers of solar filters intended for both visual and photographic use who were not operating in 1981 when I first published solar filter data in Sky and Telescope (August, 1981).Īn invitation to participate in a NATO-sponsored meeting on solar eclipse astronomy in June, 1996, prompted me to make spectrophotometric measurements of a variety of solar filter materials and assess whether these filters provide adequate protection for the eyes. Over the past five years, I have been asked to comment on the safety of using devices such as floppy disk media, multiple layers of space blanket (a very thin type of aluminized polyester), compact disks (CDs) and metal coated polyester wrappers as solar filters. This is especially true when the subject of protective filters for direct observation of the sun is raised.
Unfortunately, despite the best intentions, the news media often provide inaccurate information on safe observing techniques.
There is a great deal of interest in watching eclipses, and in the days and weeks before an eclipse occurs, there are often news stories and announcements in the media, providing information on what will happen, and how to watch the eclipse safely.