Ballistic photons are the light photons that travel through a scattering (turbid) medium in a straight line. Also known as ballistic light. If laser pulses are sent through a turbid medium such as fog or 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 ballistic photons. Photons that are slightly scattered, retaining some degree of coherence, are referred to as snake photons.
If efficiently detected, there are many applications for ballistic photons especially in coherent high resolution medical imaging systems. Ballistic scanners (using ultrafast time gates) and optical coherence tomography (OCT) (using the interferometry principle) are just two of the popular imaging systems that rely on ballistic photon detection to create diffraction-limited images. Advantages over other existing imaging modalities (e.g., ultrasounds and magnetic-resonance imaging) is that ballistic imaging can achieve a higher resolution in the order of 1 to 10 micro-meters, however it suffers from limited imaging depth. Furthermore, more scattered 'quasi-ballistic' photons are often measured as well to increase the signal 'strength' (i.e., signal-to-noise ratio).
Due to the exponential reduction (with respect to distance) of ballistic photons in a scattering medium, often image processing techniques are applied to the raw captured ballistic images, to reconstruct high quality ones. The aim of ballistic imaging modalities is to reject non-ballistic photons and to retain ballistic photons carrying useful information. To perform this task, specific characteristics of ballistic photons vs. non-ballistic photons are used, such as time of flight through coherence gated imaging, collimation, wavefront propagation and polarization.
- K. Yoo and R. R. Alfano, “Time-resolved coherent and incoherent components of forward light scattering in random media,” Optics Letters 15, 320–322 (1990).
- 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.