Visualizing Light at Trillion FPS, Camera Culture, MIT …

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The device has been developed by the MIT Media Labs Camera Culture group in collaboration with Bawendi Lab in the Department of Chemistry at MIT. A laser pulse that lasts less than one trillionth of a second is used as a flash and the light returning from the scene is collected by a camera at a rate equivalent to roughly half a trillion frames per second. However, due to very short exposure times (roughly two trillionth of a second) and a narrow field of view of the camera, the video is captured over several minutes by repeated and periodic sampling.

The new technique, which we call Femto Photography, consists of femtosecond laser illumination, picosecond-accurate detectors and mathematical reconstruction techniques. Our light source is a Titanium Sapphire laser that emits pulses at regular intervals every ~13 nanoseconds. These pulses illuminate the scene, and also trigger our picosecond accurate streak tube which captures the light returned from the scene. The streak camera has a reasonable field of view in horizontal direction but very narrow (roughly equivalent to one scan line) in vertical dimension. At every recording, we can only record a '1D movie' of this narrow field of view. In the movie, we record roughly 480 frames and each frame has a roughly 1.71 picosecond exposure time. Through a system of mirrors, we orient the view of the camera towards different parts of the object and capture a movie for each view. We maintain a fixed delay between the laser pulse and our movie starttime. Finally, our algorithm uses this captured data to compose a single 2D movie of roughly 480 frames each with an effective exposure time of 1.71 picoseconds.

Beyond the potential in artistic and educational visualization, applications include industrial imaging to analyze faults and material properties, scientific imaging for understanding ultrafast processes and medical imaging to reconstruct sub-surface elements, i.e., 'ultrasound with light'. In addition, the photon path analysis will allow new forms of computational photography, e.g., to render and re-light photos using computer graphics techniques.

Download high-resolution photos and videos

"Slow art with a trillion frames per second camera", A Velten, E Lawson, A Bardagiy, M Bawendi, R Raskar, Siggraph 2011 Talk [Link]

R Raskar and J Davis, 5d time-light transport matrix: What can we reason about scene properties, July 2007

Can you capture any event at this frame rate? What are the limitations? We can NOT capture arbitrary events at picosecond time resolution. If the event is not repeatable, the required signal-to-noise ratio (SNR) will make it nearly impossible to capture the event. We exploit the simple fact that the photons statistically will trace the same path in repeated pulsed illuminations. By carefully synchronizing the pulsed illumination with the capture of reflected light, we record the same pixel at the same exact relative time slot millions of times to accumulate sufficient signal. Our time resolution is 1.71 picosecond and hence any activity spanning smaller than 0.5mm in size will be difficult to record.

How does this compare with capturing videos of bullets in motion? About 50 years ago, Doc Edgerton created stunning images of fast-moving objects such as bullets. We follow in his footsteps. Beyond the scientific exploration, our videos could inspire artistic and educational visualizations. The key technology back then was the use of a very short duration flash to 'freeze' the motion. Light travels about a million times faster than bullet. To observe photons (light particles) in motion requires a very different approach. The bullet is recorded in a single shot, i.e., there is no need to fire a sequence of bullets. But to observe photons, we need to send the pulse (bullet of light) millions of times into the scene.

What is new about the Femto-photography approach? Modern imaging technology captures and analyzes real world scenes using 2D camera images. These images correspond to steady state light transport and ignore the delay in propagation of light through the scene. Each ray of light takes a distinct path through the scene which contains a plethora of information which is lost when all the light rays are summed up at the traditional camera pixel. Light travels very fast (~1 foot in 1 nanosecond) and sampling light at these time scales is well beyond the reach of conventional sensors (fast video cameras have microsecond exposures). On the other hand, LiDAR and Femtosecond imaging techniques such as optical coherence tomography which do employ ultra-fast sensing and laser illumination capture only the direct light (ballistic photons) coming from the scene, but ignore the indirectly reflected light. We combine the recent advances in ultra-fast hardware and illumination with a reconstruction technique that reveals unusual information.

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Visualizing Light at Trillion FPS, Camera Culture, MIT ...