Flash blindness

Flash blindness is visual impairment during and following exposure to a light flash of extremely high intensity.^[1] 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. This phenomenon is leveraged in non-lethal weapons such as flash grenades and laser dazzlers.

Cause

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 longer.

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.^[2]
• 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."^[3] The US Federal Aviation Administration in Order 7400.2F (now cancelled) defines it as "Generally, a temporary visual interference effect that persists after the source of the illumination has ceased."^[4]

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.

Potential hazards

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. Also in aviation, there is concern about laser pointers 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.^[5] "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."^[6]

Pain

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.^[7] Nonetheless, psychological pain, which activates the same pain centers in the brain^[8] and therefore is just as real, may very well be present. It can cause amplified stress levels but usually fades.

Related conditions

Welders can get a painful condition called arc eye. While it is caused by bright light as is flash blindness, the welder's arc lasts for much longer than a flash, and emits ultraviolet rays that can damage the cornea. Flash blindness, in contrast, is caused by a single very brief exposure which oversaturates the retina, and is not usually accompanied by reports of pain.

References