used in biophotonics. This process is the basis for procedures such as Raman
vibrational spectroscopy and surface-enhanced Raman scattering (SERS). Raman
scattering has a rapidly growing number of applications used for the study of the
structures and dynamic functions of biological molecules. The process also is of
importance for diagnosing and monitoring the progress of diseases such as cataract
formations, precancerous and cancerous lesions in human soft tissue, artheroscle-
rotic lesions in coronary arteries, and bone and dental pathologies.
The effects of absorption and scattering of light in tissue are considered sepa-
rately in Sects.6.2and6.3to illustrate their basic effects. However, in actual tissues
both of these effects are present simultaneously. Their combined effect is described
in Sect.6.4.
LED and laser light can interact with biological tissue through many different
mechanisms [ 1 – 11 ]. This chapter classifies light-tissue interactions into six cate-
gories that are commonly used to describe therapeutic and surgical applications. As is
discussed in Sect.6.5, these interactions can be categorized as photobiomodulation
(also called low-level light therapy), photochemical interactions, thermal interactions
(e.g., coagulation and vaporization), photoablation, plasma-induced ablation, and
photodisruption. The degree of light-tissue interaction depends on tissue character-
istics (such as the coefficients of reflection, absorption, and scattering) and the
parameters of the irradiating light. The light related parameters include wavelength,
pulse width and amplitude, pulse rate or the duration of a continuous-wave exposure,
and the focal spot size of the light beam. It is important to note that very high optical
intensities can be achieved through the combination of small spot sizes and short
pulse durations. This condition holds even for moderate pulse energies.
Next, Sect.6.6introduces the phenomenon of random interference patterns, or
specklefields, in relation to the scattering of laser light from weakly ordered media
such as tissue. The appearance of speckles arises from coherence effects in
light-tissue interactions. Among the applications areas of this effect are the study of
tissue structures and cellflow monitoring (see Chap. 10 ).
Finally, Sect.6.7presents the basic concepts offluorescence, which is the
property of certain atoms and molecules to absorb light at a particular wavelength
and, subsequently, to emit light of a longer wavelength after a short interaction
time. This physical phenomenon is widely used in a variety of biophotonics
sensing, spectroscopic, and imaging modalities.
Following the descriptions in this chapter on light-tissue interactions, subsequent
chapters discuss further applications such as biosensing, microscopy, spectroscopy,
imaging, and the use of light in microsystems, nanophotonics, and neurophotonics.
6.1 Reflection and Refraction Applications
The characteristics of reflection and refraction are discussed in Chap. 2. These two
factors are related by the Fresnel equations described in Sect.2.4. This section
describes some uses in biophotonics of reflection and refraction effects.
6 Light-Tissue Interactions 149