by: Steen Hartov
From the early days of developing nuclear weapons is was evident that one of the side effects of the blast was an extreme flash effect, so strong that the human eye would be severely affected without protection.
What is flash blindness?
The initial thermal pulse from a nuclear blast can cause eye injuries in the forms of flash blindness and retinal scarring. Flash blindness is caused by the initial brilliant flash of light produced by the nuclear detonation. This flash swamps the retina, bleaching out the visual pigments and producing temporary blindness. During daylight hours, this temporary effect may last for about 2 minutes. At night, with the pupil dilated for dark adaptation, flash blindness will affect personnel at greater ranges and for greater durations. Partial recovery can be expected in 3 to 10 minutes, though it may require 15 to 35 minutes for full night adaptation recovery. Retinal scarring is the permanent damage from a retinal burn. It will occur only when the fireball is actually in the individual's field of view and should be a relatively uncommon injury.
Impact on aircrew
It is easy to imagine how disruptive temporary blindness would be to an aircrew operating in a complex hostile environment . 10 minutes is an extremely long time not being able to see your instruments or operate your weapons systems, and even two minutes is a long time, especially if you are in the middle of a high-speed low level penetration flight. The need for protection is therefore evident.
Methods of protection
The only means of protection initially was a very dark visor combined with a monocular eye shield - an eye patch much like the ones we habitually associate with pirates and buccaneers. Later it was discovered that the protective capacity of the visor could be enhanced by applying a very thin layer of gold to the visor lens. The gold layer reduced the light transmittance of the visor to only 2 percent compared to approximately 15% for a standard neutral visor depending on type. The same sort of gold layer was applied also to astronauts’ helmets to protect them from the damaging ultraviolet rays in the 200 to 300 millimicrons range.
Needless to say the gold-plated visor lenses were very expensive. It therefore turned out to be less practical to install them for daily use where they would be easily scratched.he MIL-G-635 flash blindness goggle kit
An alternative to the gold-plated visor was developed in the form of a gold-plated lens for the well-known B-8 or M-1944 goggles. These could be worn when needed and otherwise tucked away in their protective box.
The flash blindness goggle kit (MIL-G-635) consists of one container, one complete pair of goggles, one KMU-219/P modification kit (a rubber strap), and one monocular eye shield. According to PACAF regulations MIL-G-635 goggle kits are used in multiplace aircraft where they are stored on board and distributed as required, or they are used in fighter aircraft where the PLZT goggles are too large and cumbersome.
It should be noted that the military specification is titled "MIL-G-635 Goggles, Sun, Wind And Dust". It does not say anything about the gold-plated visor lens. The MIL-G-635 is therefore only an identification of the frame used in the goggle kit, not a designation of the kit itself. Federal Stock Number of the goggles is 8475-00-133-3740, and the eye shield is FSN 8475-00-175-5323.
The electrooptical goggle
Several attempts were made over time to produce a more sophisticated form of flash protection than just a gold-plated visor. One such attempt was the electrooptical goggle under development in the early 1960s. The goggle used dual lenses in each eye piece, one fixed and one movable. Vertical metallic bars were deposited on each lens. In the open state the bars of the moving lense were superimposed with the bars of the fixed lens. In operation, the rise time of a high intensity light source was detected by a photocell which activated a small propellant piston motor which forced the moving lenses to translate such that the opaque bars now were superimposed with the clear spaces of the fixed lens. The goggle was completely self-contained and would operate four times without replacement of motors. The object was for the goggle to close within 500 microseconds and block 99.99 percent of the light transmission. AF Manual 64-4 does not say why the goggle did not pass the developmental stage but the field is open for anyone to guess. There is no doubt that it must have been difficult to guarantee that the physical movement of the lenses could meet the 500 microsecond target and it is also questionable how storage would affect the goggle.
http://www.flightgear.dk/flash.htm
