Rapatronic photographs

High-speed photography can capture athletes in action. But you need really high-speed photography to capture events like a nuclear explosion.

At the recent National Radio Science Meeting, I first encountered the idea of a rapatronic camera. These cameras have exposure times as short as 10 ns. They were developed in 1940 to capture the rapid expansion of a nuclear explosion, and they were gems of ingenuity. No mechanical shutter at that time could possibly open and close that quickly, so Harold Edgerton came up with a non-mechanical way of controlling the shutter: he put a Kerr cell between two polarizing filters oriented at 90 degrees from each other. Normally, no light would penetrate between the crossed polars. But when voltage is applied to the Kerr cell, it rotates the polarization of the incoming light 90 degrees—permitting it to pass through the second filter. By only activating the Kerr cell for a very short time, you obtain an ultra high-speed shutter.

You can view Edgerton’s hand-drawn circuit diagram to see how it worked.

Likewise, there was no way to mechanically advance the film fast enough to permit a single camera to take a sequence of high-speed shots, so in these tests they’d set up an array of the cameras, each with a slightly different delay. (Although I immediately wonder if you couldn’t have an electronically controlled refractive material behind the single lens to direct the light across a series of film segments so you wouldn’t have to physically move anything.)

The results are stunning:

At left is an explosion from Operation Tumbler-Snapper (1952), about 1 ms after detonation. The spikes along the bottom edge are evidence of the tower’s guy wires being vaporized by associated gamma rays. At right is an explosion from Operation Hardtack II (1958). This one was suspended from a balloon and the spikes here are the balloon’s mooring cables being vaporized. A beautiful ghostly array of such images is available from a google image search on “rapatronic”.

One thing I haven’t been able to determine is the etymology of “rapatronic”. It may be that Edgerton just coined it (with “rapa” for “rapid” and “tronic” for “electronic”—but that’s just a guess). Please share if anyone knows more!

You can read more about Edgerton and his various innovations aside from the rapatronic camera. Brilliant guy!

4 of 4 people learned something from this entry.

  1. Scott Van Essen said,

    January 17, 2011 at 9:08 am

    (Learned something new!)

    Amazing, stunning, and beautiful (much like some ladies I know ;) ).

    I love high-speed and slow motion imagery, getting to see physics and dynamics at a timescale that shows you structure you never would have known was there. So cool.

    Made me think of a book I’ve been re-reading that you might enjoy. It’s the Los Alamos Primer. Everything that we knew about building an atomic bomb in early 1943. There’s so much more involved than you expect, and you gain newfound respect for the people doing some of the math described with the techniques, tools, and computers of the 1940s (and by computers, I mean a room full of women with adding machines, much of the time). If you’re interested, I’d be happy to loan it to you.

  2. jim said,

    January 17, 2011 at 9:28 am

    (Learned something new!)

    Extremely cool stuff. I didn’t see any mention of the type of film they used (and whether there was specific work done to deal with x-rays, or maybe it was just slow & grainy and didn’t matter because the photos would never be enlarged much?)

  3. Terran said,

    January 18, 2011 at 2:25 pm

    (Learned something new!)

    Nifty! I had a vague sense of how they did that, but had never seen it broken down to a circuit diagram, or the trick with the Kerr cell. Nice!

    Also, if you’re interested in the photos of the nuclear explosions, for their own sake, a few years ago someone compiled a bunch of them into a book titled “100 Suns”. (I don’t recall if we flipped through a copy of that in the bookstore at the nuclear museum or not. But we have a copy at home (somewhere). Remind me sometime and I’ll pull it out for you.) They’re not all rapatronic photos, but they’re all wild and beautiful, in a terrifying sort of way.

  4. Tyestin said,

    January 22, 2011 at 5:08 pm

    (Learned something new!)

    The pictures look similar to electron microscope images of cells. Neat!

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