Ballistic photon

Ballistic Photons are the light photons that travel through the scattering (turbid) media in straight line. If laser pulses are sent through a turbid media (e.g. fog, body tissue), most of the photons are either randomly scattered or absorbed. However, across short distances, a few photons pass through the scattering medium in straight lines. These coherent photons are referred to as the ballistic photons. Note that another class of photons, those that are slightly scattered, retaining some degree of coherence, are commonly referred to as snake photons.

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If efficiently detected, there are many applications for the ballistic photons specially in coherent high resolution imaging systems. Ballistic scanners (using Ultrafast time-gates) and Optical Coherence Tomography (OCT) (using interferometry principal) are just two of the popular imaging systems that rely on ballistic photon detection to create diffraction limited images. Due to the exponential reduction (with respect to distance) of ballistic photons in a scattering medium, often image/signal processing techniques are applied to the raw captured ballistic images, to reconstruct high quality ones.

References

S. Farsiu, J. Christofferson, B. Eriksson, P. Milanfar, B. Friedlander, A. Shakouri, R. Nowak, Statistical Detection and Imaging of Objects Hidden in Turbid Media Using Ballistic Photons , Applied Optics, vol. 46, no. 23, pp. 5805-5822, Aug. 2007.