Hollywood has to resort to trickery to show moviegoers laser beams traveling through the air. That’s because the beams move too fast to be captured on film. Now a camera that records frames at a rate of 0.6 trillion every second can truly capture the bouncing path of a laser pulse.
The system was developed by researchers led by Ramesh Raskar at MIT’s Media Lab. Currently limited to a tabletop inside the group’s lab, the camera can record what happens when very short pulses of laser light—lasting just 50 femtoseconds (50,000 trillionths of a second) long—hit objects in front of them. The camera captures the pulses bouncing between and reflecting off objects.
Raskar says the new camera could be used for novel kinds of medical imaging, tracking light inside body tissue. It could also enable novel kinds of photographic manipulation. In experiments, the camera has captured frames roughly 500 by 600 pixels in size.
The fastest scientific cameras on the market typically capture images at rates in the low millions of frames per second. They work similar to the way a consumer digital camera works, with a light sensor that converts light from the lens into a digital signal that’s saved to disk.
The Media Lab researchers had to take a different approach, says Andreas Velten, a member of the research team. An electronic system’s reaction time is inherently limited to roughly 500 picoseconds, he says, because it takes too long for electronic signals to travel along the wires and through the chips in such designs. “[Our shutter speed is] just under two picoseconds because we detect light with a streak camera, which gets around the electrical problem.”
More typically used to measure the timing of laser pulses than for photography, a streak camera doesn’t need any electronics to record light. Light entering the streak camera falls onto a specialized electrode—a photocathode—that converts the stream of photons into a matching stream of electrons. That electron beam hits a screen on the back of the streak camera that’s covered with chemicals that light up wherever the beam falls. The same mechanism is at work in a traditional cathode ray tube TV set.