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Femtosecond camera can peek around a corner

Photography is all about light. Typically, a lens is used to focus the light reflected from an object, striking a light-sensitive surface inside a camera to create an image recognizable to the human eye.

Until now, that light required a direct line of sight, but at MIT Media Lab's Camera Culture group, Ramesh Raskar and Andreas Velten have devised a new way of capturing light bouncing from around a corner -- a femtophotography. In traditional photography, the speed of light is infinite and does not play a role. The Media Lab's femtocamera, however, has a finite amount of time light takes to travel from one surface to another, which provides useful information.

Using a beam-splitter and what they call a femtosecond laser to send out a laser pulse that lasts less than one-trillionth of a second, the light returning from the scene is collected by a camera at the equivalent of close to 1 trillion frames per second, analyzed, and decoded to record the location, and ultimately shape of an object outside of the direct line of sight.

Caption:Photo:Christopher Barsi and Andreas Velten / MIT Media Lab
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Determining shape and location

To create an image of the hidden object, the camera aims its laser at a nearby (nonmirrored) wall or door, bouncing the light and returning it to the sensors, which determine the shape and location of the object based on the distance and time the laser traveled.

Currently, the resolution is low, and the camera is limited to capturing centimeter-size details at a distance of a few meters. That means it can only see relatively large objects.

The laser bounces light over and over again at different angles, and eventually is able to record details of an object's position by analyzing these measurements.

Updated:Caption:Photo:MIT Media Lab
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Femtosecond laser meets picosecond-accurate camera

The imaging-camera prototype consists of a femtosecond laser illumination coupled with a picosecond-accurate camera and an inversion algorithm, resulting in what Raskar, an associate professor at the MIT Media Lab, calls a "space time impulse response" (STIR) of the scene that allows the camera to reconstruct the hidden surface.

Updated:Caption:Photo:Christopher Barsi and Andreas Velten / MIT Media Lab
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Mirrors need not apply

The principles behind this futuristic camera system are similar to how a periscope works, reflecting an image at an angle into the viewer's eye. But this digital imaging system doesn't need mirrors. Instead, it uses ordinary walls, doors, or floors to bounce or reflect bursts of laser light.

Updated:Caption:Photo:Christopher Barsi and Andreas Velten / MIT Media Lab
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From 'raw streak' to heat map

Many "raw streak" images from a pass of the laser, like the one seen here, are used to create a heat map of depth measurements, which is then further decoded to a visual representation of the hidden object. Light travels at about 1 foot/nanosecond and by sampling the light at picosecond resolution, the researchers at MIT say they can estimate shapes with centimeter accuracy.

Updated:Caption:Photo:Christopher Barsi and Andreas Velten / MIT Media Lab
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You can run, but you can't hide

Once the split beam laser was decoded, MIT was able to look around corners and "see" a three-dimensional object.

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Search and rescue applications

Potential applications for this imaging system include search and rescue efforts, in which clear lines of sight may not be possible.

Updated:Caption:Photo:Christopher Barsi and Andreas Velten / MIT Media Lab
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Safer cars, smarter surgery

Transient imaging also has significant potential benefits in collision avoidance for advanced autonomous cars or for robots. There could be medical applications as well -- for instance, allowing endoscopes to view around obstacles inside the human body.

Updated:Caption:Photo:Christopher Barsi and Andreas Velten / MIT Media Lab
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