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Optical physics

Optical physics

Camera captures at record rate

12 Aug 2008

Around 130 years ago, a wealthy businessman enlisted the expertise of British photographer Eadward Muybridge to settle, once and for all, the then-popular question of whether horses lift all four of their legs off the ground at once when they trot. Rigging together a dozen or so cameras with a neat shutter mechanism, Muybridge not only proved that trotting horses do indeed spend fleeting moments in mid-air, he demonstrated a way to expose images within two-thousandths of a second of one another.

High-speed photography has come a long way since then, and is used for more prosaic tasks, such as examining crash tests, ballistics and biomechanics. Modern systems can take photos hundreds of thousands of times per second at high resolutions, or even millions of times per second at lower resolutions. But now researchers in Jordan and the US have come up with a system that could potentially give the best of both worlds. “The combination of image quality, frame rate and frame count [our] camera system is capable of is unprecedented,” says Ala Hijazi of Hashemite University.

The main problem with achieving high frame rates in photography is that cameras are, on the whole, slow to respond. It is possible to get instantaneous images by leaving a camera exposing permanently in the dark and then only lighting up a desired object for an instant with a flash lamp. Unfortunately, if this principle is extended with more cameras and flashes, the result is a string of messy multiple exposures.

Selective wavelengths

Hijazi and his colleague Vis Madhavan of Wichita State University found that they can get around this problem by four using camera-and-flash combinations that are only sensitive to one particular wavelength — either 440, 532, 600 or 650 nm. This means that each camera can get its own instantaneous image without the interference of the other three.

“This concept allows the time separation between each image that is captured to be infinitesimally small,” explains Hijazi. “[It] results in a camera system that can capture a sequence of high-resolution images at ultra-high speeds.”

Hijazi and Madhavan’s prototype system — which employs charged-coupled devices (CCDs) for the cameras and dual-cavity, Nd:YAG lasers for the flashes — is capable of producing four images in succession at 200 MHz (200 million times per second), or eight images per second at 8 MHz (Meas. Sci. Technol. 19 085503). However, Hijazi thinks that the system should be able to take up to 100 megapixel-resolution images in the gigahertz range. This would make it faster than both electronic high-speed cameras, which can only produce frame rates of the order of 100 kHz, and “rotating drum” or “rotating mirror” cameras, which can produce low-resolution images at up to 20 MHz.

The researchers think their system will be useful in imaging materials during high-speed machining. They have already set up a company called Spectrum Optical Solutions, which they say will begin marketing the high-speed camera system in the second quarter of next year.

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