Stopping Light Cold
The cameras that for the past six years have made up Cordin’s fastest group don’t have moving parts at all—or film, for that matter. These cameras shoot at rates of up to 200 million frames per second, a rate useful, Nathan Nebeker says, for “things like the moment of combustion of aerosolized jet fuel: You’re trying to get an even wave front so that you have a controlled burn.”
In the so-called gated, intensified CCD cameras, light from the objective lens passes through a beam splitter, and multiple, identical images are formed on the photo cathodes of microchannel plate (MCP) image intensifier tubes, similar to those used in night vision goggles. Striking the front side of the intensifier, the photons free electrons inside each tube; the electrons cascade through the tube, freeing more as they proceed, until they strike a phosphorescent surface in back, which converts the electrons back into a light pattern. “The intensifier does two things,” explains Sid Nebeker. “It magnifies the light several thousand times, and it switches the light very, very fast—in a matter of nanoseconds. When you have such a short exposure you need that high light gain to get useful images.”
The images from the MCPs pass through bundles of glass fibers, stretched long and thin at one end to shrink down the picture. The small ends sit against 1-megapixel charge-coupled devices—pretty much what you’d find in a digital consumer camera—which record the images.
Popular Science asked Nathan Nebeker if he could set up a demonstration that would make nanosecond durations comprehensible in visual and intuitive terms. Why not, he suggested, shoot a series of photos of light as it travels in real time across a very short space—say, 100 feet? The fastest thing in the universe stopped dead as it makes its way across an ordinary room.
It is the first time Nebeker has attempted this, though not the first time it has been done—a single photo of light “stopped” by a different technology in the late 1940s was termed “one of the most important photographs of the century” in this magazine (for details, see www.popsci.com/exclusive). “It’s essentially a gee-whiz shot,” Nebeker admits, but not without its challenges. Light travels about a foot per nanosecond, so a 7-nanosecond laser pulse, if you were able to see it, would look like a discrete packet of photons 7 feet long.