Biophotonics_Concepts_to_Applications

abnormal tissue conditions based on the interaction of light with tissue. Absorbed
light can be converted into heat, be radiated in afluorescent process, or be con-
sumed in photochemical reactions. The strength of the absorption coefficients for
different tissue components determines how far light can penetrate into a specific
tissue at a particular wavelength and also determines how much energy a specific
tissue absorbs from a particular optical source. The degree of absorption depends on
the tissue type and in many cases is a strong function of wavelength. Light is readily
absorbed in the UV (<400 nm) and mid-IR (>2μm) regions. Thus light cannot
penetrate deeply into tissue in these spectral ranges and there is little attenuation due
to scattering. As Sect.6.2describes, there is a lower attenuation window in the
visible and near-IR spectral range running from about 400 nm to 2μm, which
allows deeper penetration of light into tissue.
Scattering of photons in tissue is another significant factor in the behavior of
light-tissue interactions, as Sect.6.3describes. Together, absorption and multiple
scattering of photons cause light beams to broaden and decay as photons travel
through tissue. Scattering dominates over absorption in the 600–1600-nm spectral
range, and both forward scattering and backscattering of incident light within tissue
are used in a variety of biophotonics applications. In a scattering process, light
interacts with tissue components so that some photons in a light beam are diverted
from their original path and sent in different directions. Typically light can penetrate
to a depth of several centimeters into a tissue. However, strong scattering of light
generally prevents observers from getting a clear image of tissue characteristics
beyond a few millimeters in depth. Scattering of photons can be either an elastic
process or an inelastic process. In the case of elastic scattering, the incident and
scattered photons have the same energy, which also means that the wavelengths do
not change during the scattering event. In this chapter,first Sect.6.3introduces the
basic concepts of elastic scattering. This material includes a discussion of Rayleigh
scattering, which is a particular category of elastic scattering. The effects of elastic
scattering are used in many biophotonics applications such as optical coherence
tomography, confocal microscopy, and elastic scattering spectroscopy.
Inelastic scattering occurs at a much lower probability than elastic scattering (by
a factor of about 10−^6 ) and involves an exchange of energy between the photon and
the scattering element. Raman scattering is the major inelastic scattering process

Incident light

Reflected light

Transmitted light

(attenuated due to

reflection,

absorption, and

Absorption scattering)

Scattering

Biological tissue

Fig. 6.1 Basic effects of
light-tissue interactions

148 6 Light-Tissue Interactions