Harold 'Doc' Edgerton was a proffessor of electrical engineering at MIT and celebrated pioneer in high speed photography. He received a bronze medal from the royal photographic society in 1934 for his work in
strobe photography, and later in 1973 the National Medal of Science.
In 1947 he partnered with Kenneth J. Germeshausen to do consulting for industrial clients. Later Herbert Grier joined them, and the company name "Edgerton, Germeshausen, and Grier" was changed to EG&G. EG&G became a prime contractor for the Atomic Energy Commission and had a major role in photographing and recording nuclear tests for the US through the fifties and sixties. For this role Edgerton and Charles Wykoff and others at EG&G developed and manufactured the Rapatronic camera.
The rapatronic (rapid action electronic) camera they developed was used to photograph the rapidly changing matter in nuclear
explosions within milliseconds of detonation, capable of capturing images of only a few millionths of a second in duration.
To overcome the speed limitation of a conventional camera's mechanical shutter, the rapatronic camera uses two polarizing filters and a Faraday cell (or Kerr cell). The two filters are mounted with their polarization angles at 90° to each other, to block all incoming light.
The Faraday cell sits between the filters and changes the polarization plane of light passing through it depending on the level of magnetic field applied, acting as a shutter when it is energized at the right time for a very
short amount of time, allowing the film to be properly exposed.
In magneto-optical shutters, the active material of the Faraday cell (dense magnetically reactive glass) is located inside an electromagnetic coil, formed by a few loops of thick wire. The coil is powered from a pulse forming network by discharging a high-voltage capacitor into the coil via a trigatron or a thyratron switch.
Rapatronic camera (The Smithsonian)
In electro-optical shutters, the active material is a liquid, typically nitrobenzene, located in a cell between two electrodes. A brief impulse of high voltage is then applied to rotate the polarization of the passing light. The camera was triggered by a photocell which detected the initial x-rays from the weapon casing. This 'zero time' could
then be delayed by the required amount before activating the magneto-optic shutter.
Up to twelve cameras were deployed, with each camera carefully timed to record sequentially. Each camera was capable of recording only one exposure on a single sheet of film. Therefore, in order to create time-lapse sequences, banks of four to ten cameras were set up to take photos in rapid succession. The average exposure time used was three microseconds.
The resulting extraordinary photographs revealed intricate details of the first instant of an atomic explosion, including a
few surprises such as irregular “mottling” caused primarily by variations in the density of the bomb’s casing as it splashed against the initial shock-front. It also showed the “rope trick effect,” where the rapid vaporization of support cables caused spikes to emanate from the bottom of an explosion.
These photographs were essential in the analysis of the tests. Primarily by measuring the size of the initial fireball to ascertain
the yield of the blast. This information could be used by the weapon designers to calculate the efficiency of the weapon and subsequently the success of the design. The fissionable materials used in nuclear weapons are
extremely expensive, so greater efficiency could be translated into less material used, or a higher yield for the same quantity of material.
Below, Charles Wyckoff one of the developers of the Rapatronic camera, explains the technology he and Dr. Edgerton created.
