Flash blindness is visual impairment during and following exposure to a light flash of extremely high intensity. It may last for a few seconds to a few minutes.
For example, in everyday life, the subject of a flash photograph can be temporarily flash blinded. The bright light overwhelms the eye and only gradually fades. A bright spot or spots may be seen for many minutes.
Flash blindness is caused by bleaching (oversaturation) of the retinal pigment. As the pigment returns to normal, so too does sight. In daylight the eye's pupil constricts, thus reducing the amount of light entering after a flash. At night, the dark-adapted pupil is wide open so flash blindness has a greater effect and lasts for a longer time.
Temporary vs. permanent
Is flash blindness temporary or permanent?
- Some sources such as NATO and the U.S. Department of Defense state that "flash blindness" can be temporary or permanent.
- Other sources restrict the use of the word to temporary, reversible vision loss: "...These are, in order of increasing brightness: dazzle, after image formation, flash blindness, and irreversible damage." The U.S. Federal Aviation Administration in Order 7400.2f defines it as "Generally, a temporary visual interference effect that persists after the source of the illumination has ceased."
Because there appears to be no consensus definition, one should be especially clear about which sense(s) of the phrase are meant. For example, using the phrase "temporary flash blindness" when discussing everyday flash photography emphasizes that the condition will disappear without ill effect.
Because vision loss is sudden and takes time to recover, flash blindness can be hazardous. At some sporting events such as figure skating, fans are cautioned to not use flash photography so as to avoid distracting or disorienting the athletes. In aviation, pilots are trained to recover from bright nearby lightning flashes. Also in aviation, there is concern about lasers and bright searchlights causing temporary flash blindness and other vision-distracting effects in pilots who are in critical phases of flight such as approach and landing.
The bright initial flash of a nuclear weapon is the first indication of a nuclear explosion, traveling faster than the blast wave or sound wave. "A 1-megaton explosion can cause flash blindness at distances as great as 13 miles on a clear day, or 53 miles on a clear night. If the intensity is great enough, a permanent retinal burn will result."
It is unclear whether pain is directly associated with flash blindness. Reaction to flash blindness can be discomforting and disorienting. The retina has no pain receptors, so reports of pain may be due to more psychological reactions.
Welders can get a painful condition called arc eye. While caused by bright light, the welder's flash is of longer duration and emits ultraviolet rays that can affect the cornea. Flash blindness, in contrast, can be caused by a single very brief exposure which oversaturates the retina, and is not usually accompanied by reports of pain.
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