Scatterings

Revealing Hidden Objects with Ultrafast Imaging

Yvonne Carts-Powell

Scatterings image

A 3-D spade (left) is rendered in 2-D from thermal infrared energy emitted by the object. The different colors represent areas of higher and lower temperature. The image on right is the optically reconstructed object. The gray ground plane was added to provide context.

A system that detects the 3-D geometry of hidden shapes was developed by researchers from the Massachusetts Institute of Technology (MIT), Harvard University, the University of Wisconsin and Rice University (Opt. Express 20(17) 19096). Although time-of-flight imaging has been used for years for objects in direct view of the camera, researchers showed that a 2-D streak camera can discover and reconstruct the geometry of objects without a direct line of sight from the camera.
 
Speed is crucial. They used a mode-locked Ti:Sapphire laser to generate 50-fs infrared pulses  with a repetition rate of 75 MHz. The pulse illuminates a diffusing surface, and the scattered light hits the hidden object. Some of the light scattered from the hidden object returns to the diffuser, and back (much attenuated) to the streak camera. The streak camera has a 1-D field-of-view, imaging a line in the scene. It provides a 2-D image in which one dimension corresponds to space and the other to time. Thus, the camera records the length of the trip taken by the photons. From this information, the method can reconstruct the 3-D shape of objects. The system is limited by signal-to-noise, since much of the signal is lost due to scattering. Results are also dependent on the presence of a good diffuser.
 
"It's ideal for use in nearly any disaster-type situation," says lead author Otkrist Gupta, "especially fires, in which you need to find out what's going on inside and around corners—but don't want to risk sending someone inside because of dangerous or hazardous conditions." More immediately, the system could be used for biological imaging.
 


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