Chapter 6
Light-Tissue Interactions
Abstract A fundamental challenge in biophotonics is to understand the interaction
of light with multilayered, multicomponent, and optically inhomogeneous biological
tissues. The effects of light-tissue interactions include reflection and refraction when
light encounters different tissue types, absorption of photon energy, and multiple
scattering of photons. Light absorption determines how far light can penetrate into a
specific tissue. It depends strongly on wavelength and is important in the diagnosis
and therapy of abnormal tissue conditions. Scattering of photons in tissue is another
significant factor in light-tissue interactions. Together, absorption and multiple
scattering of photons cause light beams to broaden and decay as photons travel
through tissue. Light can interact with biological tissue through many different
mechanisms, including photobiomodulation, photochemical interactions, thermal
interactions (e.g., coagulation and vaporization), photoablation, plasma-induced
ablation, and photodisruption. Two key phenomena used in tissue analyses are
random interference patterns or specklefields and the principles offluorescence.
The interaction of light with biological tissues is a complex process because the
constituent tissue materials are multilayered, multicomponent, and optically inho-
mogeneous. As shown in Fig.6.1, the basic effects of light-tissue interactions include
reflection at a material interface, refraction when light enters a tissue structure that has
a different refractive index, absorption of photon energy by the material, and multiple
scattering of photons in the material. Owing to the fact that diverse intermingled
biological tissue components have different optical properties, then along some path
through a given tissue volume, various physical parameters (for example, the
refractive index, the absorption coefficients, and the scattering coefficients) can vary
continuously or undergo abrupt changes at material boundaries, for example, at
interfaces between soft biological tissue and organs or bone tissue.
In this chapterfirst Sect.6.1 describes some basic reflection and refraction
phenomena that occur when light impinges on tissue. This description expands on
the discussion in Sect.2.4by considering applications of reflection and refraction in
biophotonics. Following this discussion, Sect.6.2addresses light absorption, which
is a highly important factor in the diagnosis of tissue properties and in the therapy of
©Springer Science+Business Media Singapore 2016
G. Keiser,Biophotonics, Graduate Texts in Physics,
DOI 10.1007/978-981-10-0945-7_6
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