Femtosecond Photography

7/29/2019 Femtosecond Photography

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Femtophotography(Trillion frames per second):

Capturing echoes of light around a

corner

Ashar A Z

CPMU Seminar

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Theme


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Plan of talk

Fast photography

Streak camera: Principle and operation

Imaging around a corner : Steps involved

Experimental setup

Geometric reconstruction using streak camera

5D Light propagation, model and surface reconstruction

3D Reconstruction process

Applications

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Fast photography

nanosecond exposuremagneto optic shutter

Raptronic camera used for collecting nuclear explosion after 2 msDr. Harold Edgerton, Germeshausen and Grier

BVd Eadweard Muybridge,Source: wikipedia

4Source: http://webmuseum.mit.edu


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Femtosecond laser Kerr lens mode locked Ti: Sapphire laser=795 nmRepetitions rate = 75 MHz, ie pulse repeats in every 13.3 ns

Streak cameraExposure time =100 ms , ie 7.5 million reading

Computation for reconstruction of image

Components of femtophotographyLooking around a corner to see a hidden object

fs50

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Pico10-12 sec

Velocity of light = 3 10 8 m/sDistance travelled by light in 2 ps ~ 610 -4 meter ~ 0.6 mm

Femtophotography

6Source: http://web.media.mit.edu/~raskar/cornar/


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Streak camera: Working principle

Source: Photonic Device Dynamics Group website,Tyndall National institute.7


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(a)

(b)

=888 nm

Time =0.5 ns

A streak camera will stretch information

in time.

Ultrafast, femtosecond time resolution

3D

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Time resolved photoluminescence

Pulsedlaser

Spectrograph

Streak camera

Sample

Source: Photonic Device Dynamics Group website,Tyndall National institute.


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Streak camera: Image formation

s

Point source of light

Spherical wavefront contribute tohyperbola in space time streakimage.


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Why streak camera ?

100 cm

1 cm

P1

P2

Velocity of light , c 3x10 10 cm/s

3x10 10 cm 1 second

0.05 cm 1.6 x 10 -12 seconds,

PICO SECONDS

Traditional cameras are limited by temporal resolution

Detector

Materials transport limitations


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Imaging around a corner

Phase I : Data acquisition 60 laser position on wall with 100 images for each position images taken at one position are overlayed to reduce the noise.

Optical imaging + Computational reconstruction

Phase II : Data processing Streak images are loaded, and corrected for intensity andspatiotemporal jitter.

Phase III: 3D reconstruction Corrected streak images are used in back projection algorithmto reconstruct the image


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Phase I:Experimental setup

Calibrationspot

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Source : Nature video


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Foci of ellipse

Back projection of points

Location on the hidden surfacepatch which contributed to pixelsp,q and r.

No correspondenceWe dont know whether or notlight detected at two pixels camefrom the same point on hidden

object 13

Phase II: Geometric reconstruction usingstreak camera


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22 cm2

Occluder

Diffuserwall

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Scale bar 2 cm


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Phase III: Modeling light pulse propagation

5D light transport.

4 segments for optical path

1

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4 Segments 1 and 4 directed segments.

data received bycamera Cat time tat pointp

),( tpIC

)),((),( wCBLtwHItwI CR

Primary scattering problem.

data received surfaceat time tat pointp

),( twIR

H(w) is the projective transformation (homography) mapping coordinates

onR to camera co-ordinates 15

Time compensation for segment (1) and (4)


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Measuring distance in time units, setting c=1

SSc

c

R sddsItrr

twI

2

2),()(1),(

S lclc

R sdrrtrr

ItwI 222

)(11

),(

)/()(),( 2llS rrIsI sLrl

Generating streak image

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emitter sending pulses at time),( sIS


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Hyperbolic contribution

For fixedL and w at time t, t= rc + rl

In (w,t) space

222 ),()()( yxzvyuxrrt cl , a prolate spheroid

Each point of surfaceSwill contribute a hyperboloid to the imageIR(u , v , t).

Hyperboloid shape depends on depthz(x , y) and will be shifted along t axis

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Back projection: Principle of ComputedTomography

Each sample acquired in a CTsystem is equal to the sum of theimage values along a ray pointing tothat sample

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Absorption

http://www.dnatube.com

x


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Backprojection is formed bysmearing eachview back through the image in the

direction it was originally acquired

Filtered back projection

Digital Signal Processing: A Practical Guide for Engineers and Scientist By Steven W. Smith 19


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Voxel Grid setup

Down sampling(optional)

Filtering and threshold

Back projection

Using time of flight(ToF) method

3D Reconstruction

A voxel v can contribute to streakcamera pixel only if it satisfy ToF criteria.

Heat mapsum is overall contributing

pixelsp, andIPis the intensitymeasured at that pixel.

PP

ILvwvvH

)()(


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Applications

23Source: http://web.media.mit.edu/~raskar/cornar/


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Thank you

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Documents

Femtosecond Photography