Ballistic and Quasi-Ballistic Radiation

doi:10.1117/3.2624517.ch6

Book: Light Propagation through Biological Tissue and Other Diffusive Media: Theory, Solutions, and Validation, Second Edition

Author(s): Fabrizio Martelli, Tiziano Binzoni, André Liemert, Samuele Del Bianco, Alwin Kienle

Published: 2022

https://doi.org/10.1117/3.2624517.ch6

Author Affiliations +

Abstract

At the opposite of the diffusive regime of light propagation in scattering media, there is the ballistic regime. The intent of this chapter is to provide the basic solutions for the ballistic and quasi-ballistic regime of propagation and a model for their detection in a collimated transmittance configuration. The ballistic regime shows complementary characteristics compared to the diffusive regime. The analysis of ballistic radiation is paradigmatic for understanding the complementary peculiarities of the diffusive regime and to describe the transition from ballistic light to diffusive. In the diffusive regime, radiance expresses a quasi-isotropic angular distribution, but in the ballistic regime, photons move along their initial direction of propagation. These complementary characteristics of ballistic and diffusive photons reveal the need of studying ballistic solutions for a complete understanding of photon migration in radiative transfer processes where often ballistic and diffusive photons coexist. A distinction between ballistic and diffusive photons is particularly required in applications based on ballistic detection. Ballistic light, by definition, is directly linked to the extinction coefficient of the medium, and for imaging purposes, the photons’ paths can thus be precisely identified without resorting to complex reconstruction techniques. Therefore, measurements of ballistic light are largely used in spectrophotometric applications aimed to extract information from the extinction coefficient of the medium and in imaging applications for media with small optical thicknesses (see Chapters 7 and 8 of Ref. 1 for some examples of applications). For these reasons, the study of ballistic radiation is an important part of photon migration in scattering media.